NCERT Class 9 Science Solutions: Atoms and Molecules

Question:

Calculate the molecular mass of CH₄.

Concept in a Minute:

Molecular mass is the sum of the atomic masses of all the atoms in a molecule. Atomic mass is the mass of an atom of a chemical element, usually expressed in atomic mass units (amu). To calculate molecular mass, we need to know the chemical formula of the substance and the atomic masses of each element present.

Explanation:

To calculate the molecular mass of methane (CH₄), we need to find the atomic masses of carbon (C) and hydrogen (H) and then sum them up according to the chemical formula.

1. Identify the elements and their counts in the molecule:
The chemical formula for methane is CH₄.
This means there is 1 atom of Carbon (C) and 4 atoms of Hydrogen (H).

2. Find the atomic masses of each element:
From the periodic table, the approximate atomic mass of Carbon (C) is 12.011 amu.
From the periodic table, the approximate atomic mass of Hydrogen (H) is 1.008 amu.

3. Calculate the contribution of each element to the molecular mass:
Contribution of Carbon = (Number of Carbon atoms) × (Atomic mass of Carbon)
Contribution of Carbon = 1 × 12.011 amu = 12.011 amu

Contribution of Hydrogen = (Number of Hydrogen atoms) × (Atomic mass of Hydrogen)
Contribution of Hydrogen = 4 × 1.008 amu = 4.032 amu

4. Sum the contributions to get the molecular mass:
Molecular mass of CH₄ = Contribution of Carbon + Contribution of Hydrogen
Molecular mass of CH₄ = 12.011 amu + 4.032 amu
Molecular mass of CH₄ = 16.043 amu

Therefore, the molecular mass of CH₄ is approximately 16.043 amu. Often, for high school calculations, rounded atomic masses are used (e.g., C = 12 amu, H = 1 amu), which would give a molecular mass of 1 × 12 + 4 × 1 = 16 amu. Using more precise values gives a slightly more accurate result.

Final Answer: The molecular mass of CH₄ is 16.043 amu.

Question:

In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.

Sodium carbonate + acetic acid → sodium acetate + carbon dioxide + water

Concept in a Minute:

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. This means that the total mass of the reactants must be equal to the total mass of the products.

Explanation:

To show that the observations are in agreement with the law of conservation of mass, we need to calculate the total mass of the reactants and the total mass of the products and then compare them.

Step 1: Identify the reactants and their masses.
Reactants are sodium carbonate and acetic acid.
Mass of sodium carbonate = 5.3 g
Mass of acetic acid = 6 g

Step 2: Calculate the total mass of the reactants.
Total mass of reactants = Mass of sodium carbonate + Mass of acetic acid
Total mass of reactants = 5.3 g + 6 g

Step 3: Identify the products and their masses.
Products are sodium acetate, carbon dioxide, and water.
Mass of sodium acetate = 8.2 g
Mass of carbon dioxide = 2.2 g
Mass of water = 0.9 g

Step 4: Calculate the total mass of the products.
Total mass of products = Mass of sodium acetate + Mass of carbon dioxide + Mass of water
Total mass of products = 8.2 g + 2.2 g + 0.9 g

Step 5: Compare the total mass of reactants and the total mass of products.
If the total mass of reactants is equal to the total mass of products, then the observations are in agreement with the law of conservation of mass.

Calculation:
Total mass of reactants = 5.3 g + 6 g = 11.3 g
Total mass of products = 8.2 g + 2.2 g + 0.9 g = 11.3 g

Conclusion:
Since the total mass of reactants (11.3 g) is equal to the total mass of products (11.3 g), these observations are in agreement with the law of conservation of mass.

Question:

Calculate the molecular mass of C₂H₄.

Concept in a Minute:

To calculate the molecular mass of a compound, you need to sum the atomic masses of all the atoms present in its chemical formula. Atomic masses can be found on the periodic table.

Explanation:

The chemical formula for ethylene is C₂H₄.
This means there are 2 carbon atoms and 4 hydrogen atoms in one molecule of C₂H₄.

To find the molecular mass, we need the atomic masses of carbon (C) and hydrogen (H).
From the periodic table:
Atomic mass of Carbon (C) = approximately 12.011 atomic mass units (amu)
Atomic mass of Hydrogen (H) = approximately 1.008 atomic mass units (amu)

Now, calculate the total mass contributed by each element:
Mass of 2 Carbon atoms = 2 * (atomic mass of C) = 2 * 12.011 amu = 24.022 amu
Mass of 4 Hydrogen atoms = 4 * (atomic mass of H) = 4 * 1.008 amu = 4.032 amu

Finally, add the masses of all the atoms to get the molecular mass of C₂H₄:
Molecular mass of C₂H₄ = (Mass of 2 Carbon atoms) + (Mass of 4 Hydrogen atoms)
Molecular mass of C₂H₄ = 24.022 amu + 4.032 amu
Molecular mass of C₂H₄ = 28.054 amu

Therefore, the molecular mass of C₂H₄ is approximately 28.054 amu.
Often, for high school calculations, rounded atomic masses are used:
Atomic mass of Carbon (C) = 12 amu
Atomic mass of Hydrogen (H) = 1 amu

Using rounded values:
Mass of 2 Carbon atoms = 2 * 12 amu = 24 amu
Mass of 4 Hydrogen atoms = 4 * 1 amu = 4 amu
Molecular mass of C₂H₄ = 24 amu + 4 amu = 28 amu

Question:

Calculate the molar mass of the following substance:

Sulphur molecule, S₈

Concept in a Minute:

To calculate the molar mass of a substance, you need to:
1. Identify the chemical formula of the substance.
2. Find the atomic mass of each element present in the substance.
3. Multiply the atomic mass of each element by the number of atoms of that element in the molecule.
4. Sum up the masses of all the atoms to get the molecular mass.
5. The molar mass is numerically equal to the molecular mass but expressed in grams per mole (g/mol).

Explanation:

The question asks to calculate the molar mass of a sulphur molecule, S₈.

Step 1: Identify the chemical formula.
The chemical formula is given as S₈. This means a sulphur molecule consists of 8 sulphur atoms.

Step 2: Find the atomic mass of sulphur.
The atomic mass of sulphur (S) is approximately 32.07 atomic mass units (amu). For calculations, we often use rounded values, so we can consider it as 32 g/mol.

Step 3: Calculate the total mass of sulphur atoms in the molecule.
Since there are 8 sulphur atoms in the S₈ molecule, the total mass contributed by sulphur is:
Mass of S₈ = (Number of S atoms) × (Atomic mass of S)
Mass of S₈ = 8 × 32.07 g/mol

Step 4: Calculate the molar mass.
Molar mass of S₈ = 8 × 32.07 g/mol = 256.56 g/mol.
Rounding to a more commonly used significant figure, we can say the molar mass is approximately 256 g/mol.

Therefore, the molar mass of the sulphur molecule, S₈, is 256.56 g/mol.

Question:

Calculate the molecular mass of NH_3.

Concept in a Minute:

Molecular mass is the sum of the atomic masses of all the atoms in a molecule. Atomic mass is the mass of an atom of a chemical element. Standard atomic weights are typically used for calculations.

Explanation:

To calculate the molecular mass of ammonia (NH3), we need to find the atomic masses of nitrogen (N) and hydrogen (H) and then sum them up according to the molecular formula.

Step 1: Identify the elements present in the molecule and their number of atoms.
The molecule is NH3.
It contains one atom of Nitrogen (N) and three atoms of Hydrogen (H).

Step 2: Find the atomic mass of each element.
From the periodic table, the approximate atomic mass of Nitrogen (N) is 14.01 atomic mass units (amu).
The approximate atomic mass of Hydrogen (H) is 1.01 atomic mass units (amu).

Step 3: Multiply the atomic mass of each element by the number of atoms of that element in the molecule.
Mass contributed by Nitrogen = (1 atom of N) × (14.01 amu/atom of N) = 14.01 amu
Mass contributed by Hydrogen = (3 atoms of H) × (1.01 amu/atom of H) = 3.03 amu

Step 4: Sum the masses contributed by each element to find the molecular mass.
Molecular mass of NH3 = Mass contributed by Nitrogen + Mass contributed by Hydrogen
Molecular mass of NH3 = 14.01 amu + 3.03 amu
Molecular mass of NH3 = 17.04 amu

Therefore, the molecular mass of NH3 is 17.04 amu.

Question:

Calculate the molecular mass of CH₃OH.

Concept in a Minute:

Molecular mass is the sum of the atomic masses of all atoms in a molecule. Atomic masses of elements can be found on the periodic table.

Explanation:

To calculate the molecular mass of CH3OH, we need to find the atomic masses of each element present and the number of atoms of each element.

1. Identify the elements present: Carbon (C), Hydrogen (H), and Oxygen (O).
2. Determine the number of atoms of each element from the chemical formula CH3OH:
* Carbon (C): 1 atom
* Hydrogen (H): 3 (from CH3) + 1 (from OH) = 4 atoms
* Oxygen (O): 1 atom
3. Find the atomic masses of each element from the periodic table (approximate values):
* Atomic mass of Carbon (C) ≈ 12.01 u
* Atomic mass of Hydrogen (H) ≈ 1.01 u
* Atomic mass of Oxygen (O) ≈ 16.00 u
4. Calculate the total mass contributed by each element:
* Mass of Carbon = 1 atom * 12.01 u/atom = 12.01 u
* Mass of Hydrogen = 4 atoms * 1.01 u/atom = 4.04 u
* Mass of Oxygen = 1 atom * 16.00 u/atom = 16.00 u
5. Sum the masses of all elements to get the molecular mass:
Molecular mass of CH3OH = Mass of Carbon + Mass of Hydrogen + Mass of Oxygen
Molecular mass of CH3OH = 12.01 u + 4.04 u + 16.00 u
Molecular mass of CH3OH = 32.05 u

Therefore, the molecular mass of CH3OH is approximately 32.05 atomic mass units (u).

Question:

Calculate the molecular mass of CO₂.

Concept in a Minute:

To calculate the molecular mass of a compound, you need to sum the atomic masses of all the atoms present in its molecule. You will need a periodic table to find the atomic masses of individual elements.

Explanation:

The chemical formula for carbon dioxide is CO₂. This means one molecule of carbon dioxide contains one atom of carbon (C) and two atoms of oxygen (O).

Step 1: Find the atomic mass of carbon (C).
From the periodic table, the atomic mass of carbon is approximately 12.011 atomic mass units (amu).

Step 2: Find the atomic mass of oxygen (O).
From the periodic table, the atomic mass of oxygen is approximately 15.999 atomic mass units (amu).

Step 3: Calculate the total mass contributed by carbon atoms.
Since there is 1 carbon atom, the mass contributed by carbon is 1 × 12.011 amu = 12.011 amu.

Step 4: Calculate the total mass contributed by oxygen atoms.
Since there are 2 oxygen atoms, the mass contributed by oxygen is 2 × 15.999 amu = 31.998 amu.

Step 5: Sum the masses to find the molecular mass of CO₂.
Molecular mass of CO₂ = (Mass of carbon) + (Mass of oxygen)
Molecular mass of CO₂ = 12.011 amu + 31.998 amu
Molecular mass of CO₂ = 44.009 amu

For typical high school calculations, rounding to two decimal places is usually sufficient, so:
Molecular mass of CO₂ ≈ 44.01 amu.

Question:

Write down the name of a compound represented by the following formula:

Al₂(SO₄)₃

Concept in a Minute:

Chemical nomenclature of ionic compounds. Understanding how to name compounds based on their constituent ions. Specifically, recognizing the ions present in the given formula and knowing their common names.

Explanation:

The given chemical formula is Al₂(SO₄)₃.
This formula represents an ionic compound formed between aluminum ions and sulfate ions.
First, identify the cation and the anion.
The symbol ‘Al’ represents the element Aluminum. Aluminum is a metal and typically forms a cation with a charge of +3, so it is the aluminum ion, Al³⁺.
The group of atoms ‘SO₄’ represents the sulfate polyatomic ion, which has a charge of -2, so it is the sulfate ion, SO₄²⁻.
To name an ionic compound, we write the name of the cation followed by the name of the anion.
In this case, the cation is aluminum ion and the anion is sulfate ion.
Therefore, the name of the compound Al₂(SO₄)₃ is Aluminum sulfate.
The subscripts in the formula indicate the ratio of ions needed to form a neutral compound. In Al₂(SO₄)₃, there are 2 aluminum ions (2 x +3 = +6 charge) and 3 sulfate ions (3 x -2 = -6 charge), resulting in a neutral compound. However, for naming purposes, we only use the names of the ions, not the number of atoms.

The name of the compound represented by the formula Al₂(SO₄)₃ is Aluminum sulfate.

Question:

Write the chemical formula of the Magnesium chloride.

Concept in a Minute:

Chemical formulas represent the composition of compounds using element symbols and subscripts. To write the formula for an ionic compound like magnesium chloride, you need to know the charges of the ions involved and ensure the overall compound is electrically neutral. Magnesium is a metal in Group 2 of the periodic table, typically forming a +2 ion. Chlorine is a nonmetal in Group 17, typically forming a -1 ion.

Explanation:

Magnesium (Mg) is in Group 2 of the periodic table and tends to lose two electrons to form a magnesium ion with a charge of +2 (Mg^2+). Chlorine (Cl) is in Group 17 and tends to gain one electron to form a chloride ion with a charge of -1 (Cl^-). To form a neutral compound, the total positive charge must equal the total negative charge. Therefore, we need two chloride ions (each with a -1 charge) to balance the charge of one magnesium ion (with a +2 charge). This means the ratio of magnesium ions to chloride ions is 1:2. The chemical formula is written by placing the symbol of the cation (positive ion) first, followed by the symbol of the anion (negative ion), with subscripts indicating the number of each ion needed for neutrality. Thus, the chemical formula for magnesium chloride is MgCl2.

Question:

Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Concept in a Minute:

Dalton’s atomic theory proposes fundamental ideas about atoms. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.

Explanation:

Dalton’s atomic theory has several postulates. One of these postulates states that “Atoms can neither be created nor destroyed in a chemical reaction.” This is directly derived from the law of conservation of mass. The law of conservation of mass, observed experimentally, means that the total mass of reactants before a chemical reaction is equal to the total mass of products after the reaction. If atoms could be created or destroyed, then the mass would not be conserved. Therefore, Dalton’s postulate that atoms are indivisible and indestructible in chemical reactions is a direct consequence and explanation of the experimentally observed law of conservation of mass.

The specific postulate of Dalton’s atomic theory that is the result of the law of conservation of mass is: “Atoms of a given element are identical in mass and properties.” (This is incorrect interpretation).

The correct postulate that is the result of the law of conservation of mass is: “Atoms can neither be created nor destroyed in a chemical reaction.”

Question:

Write down the name of a compound represented by the following formula:

CaCl₂

Concept in a Minute:

Ionic compounds are named by combining the name of the cation (positive ion) and the anion (negative ion). For binary ionic compounds (containing two elements), the cation name is the name of the metal element, and the anion name is the name of the nonmetal element with its ending changed to “-ide”.

Explanation:

The formula CaCl₂ represents a compound formed between calcium (Ca) and chlorine (Cl).
Calcium (Ca) is an element from Group 2 of the periodic table, and it forms a cation with a charge of +2, which is written as Ca²⁺. The name of this cation is calcium.
Chlorine (Cl) is an element from Group 17 of the periodic table, and it forms an anion with a charge of -1, which is written as Cl⁻. When chlorine forms an anion, its name changes to chloride.
In the compound CaCl₂, one calcium ion (Ca²⁺) is combined with two chloride ions (2 Cl⁻) to achieve electrical neutrality (total positive charge +2, total negative charge -2).
Therefore, the name of the compound is formed by combining the name of the cation (calcium) and the name of the anion (chloride).

The name of the compound CaCl₂ is Calcium chloride.

Question:

Write the chemical formula of the Calcium oxide.

Concept in a Minute:

Chemical Formula Writing Rules:
1. Identify the elements involved.
2. Determine the valency (combining capacity) of each element.
3. Criss-cross the valencies to determine the subscripts in the chemical formula.
4. Simplify the subscripts if they have a common factor.

Explanation:

Calcium is an element belonging to Group 2 of the periodic table, which means it typically has a valency of +2. Oxide refers to the element oxygen, which typically has a valency of -2.

To write the chemical formula for Calcium oxide, we combine Calcium (Ca) and Oxygen (O).

Valency of Calcium (Ca) = 2
Valency of Oxygen (O) = 2

Using the criss-cross method:
The valency of Calcium (2) becomes the subscript for Oxygen.
The valency of Oxygen (2) becomes the subscript for Calcium.

This gives us a preliminary formula of Ca₂O₂.

Since the subscripts 2 and 2 have a common factor of 2, we simplify them by dividing both by 2.
2 ÷ 2 = 1
2 ÷ 2 = 1

So, the simplest ratio is Ca₁O₁.

By convention, when the subscript is 1, it is not written. Therefore, the chemical formula of Calcium oxide is CaO.

The final answer is $\boxed{CaO}$.

Question:

Calculate the molar mass of the following substance:

Nitric acid, HNO₃

Concept in a Minute:

To calculate the molar mass of a compound, you need to sum the atomic masses of all the atoms present in its chemical formula. Atomic masses of elements can be found on the periodic table.

Explanation:

The chemical formula for nitric acid is HNO₃. This means one molecule of nitric acid contains:
– 1 atom of Hydrogen (H)
– 1 atom of Nitrogen (N)
– 3 atoms of Oxygen (O)

To find the molar mass of HNO₃, we will add the atomic masses of each element, taking into account the number of atoms of each element in the formula.

From the periodic table, the approximate atomic masses are:
– Atomic mass of Hydrogen (H) = 1.008 g/mol
– Atomic mass of Nitrogen (N) = 14.007 g/mol
– Atomic mass of Oxygen (O) = 15.999 g/mol

Now, we calculate the molar mass of HNO₃:
Molar mass of HNO₃ = (1 × Atomic mass of H) + (1 × Atomic mass of N) + (3 × Atomic mass of O)
Molar mass of HNO₃ = (1 × 1.008 g/mol) + (1 × 14.007 g/mol) + (3 × 15.999 g/mol)
Molar mass of HNO₃ = 1.008 g/mol + 14.007 g/mol + 47.997 g/mol
Molar mass of HNO₃ = 63.012 g/mol

Therefore, the molar mass of nitric acid (HNO₃) is 63.012 g/mol.

Question:

What are polyatomic ions? Give examples.

Concept in a Minute:

Ions are charged species. Polyatomic ions are ions that consist of more than one atom covalently bonded together, carrying an overall charge. To answer this, you need to understand what an ion is and the concept of covalent bonding.

Explanation:

A polyatomic ion is a group of atoms that are covalently bonded together and collectively carry an electrical charge. This charge can be positive or negative. These ions act as a single unit in chemical reactions and in ionic compounds.

Examples of polyatomic ions:
Sulfate ion (SO4²⁻): This ion consists of one sulfur atom and four oxygen atoms, with an overall charge of -2.
Nitrate ion (NO3⁻): This ion is composed of one nitrogen atom and three oxygen atoms, carrying a charge of -1.
Ammonium ion (NH4⁺): This ion is made up of one nitrogen atom and four hydrogen atoms, with an overall charge of +1.
Carbonate ion (CO3²⁻): This ion comprises one carbon atom and three oxygen atoms, and it has a charge of -2.
Phosphate ion (PO4³⁻): This ion consists of one phosphorus atom and four oxygen atoms, with an overall charge of -3.

Question:

A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

Concept in a Minute:

Percentage composition by weight of an element in a compound is calculated by dividing the mass of the element by the total mass of the compound and multiplying by 100.

Explanation:

Step 1: Identify the given information.
Mass of the compound sample = 0.24 g
Mass of boron in the sample = 0.096 g
Mass of oxygen in the sample = 0.144 g

Step 2: Calculate the percentage of boron by weight.
Percentage of boron = (Mass of boron / Mass of compound) * 100
Percentage of boron = (0.096 g / 0.24 g) * 100

Step 3: Perform the calculation for boron percentage.
Percentage of boron = 0.4 * 100 = 40%

Step 4: Calculate the percentage of oxygen by weight.
Percentage of oxygen = (Mass of oxygen / Mass of compound) * 100
Percentage of oxygen = (0.144 g / 0.24 g) * 100

Step 5: Perform the calculation for oxygen percentage.
Percentage of oxygen = 0.6 * 100 = 60%

Step 6: Verify the results. The sum of the percentage compositions should be 100%.
40% (Boron) + 60% (Oxygen) = 100%

Final Answer: The percentage composition of the compound by weight is 40% boron and 60% oxygen.

Question:

Write down the formula of aluminium chloride.

Concept in a Minute:

Understanding chemical nomenclature, specifically the naming and formula writing of ionic compounds, requires knowledge of the valencies (or common charges) of the constituent elements and the principle of charge neutrality in forming a stable compound.

Explanation:

To write the formula of aluminium chloride, we need to identify the elements involved and their respective valencies.
Aluminium (Al) is an element in Group 13 of the periodic table and typically forms a cation with a charge of +3 (Al³⁺).
Chlorine (Cl) is a halogen in Group 17 of the periodic table and typically forms an anion with a charge of -1 (Cl⁻).

For a compound to be electrically neutral, the total positive charge must equal the total negative charge. In aluminium chloride, we have Al³⁺ and Cl⁻. To balance the charges, we need three chloride ions (each with a -1 charge) to counteract the +3 charge of one aluminium ion.

Therefore, the ratio of aluminium ions to chloride ions is 1:3.
This leads to the chemical formula: AlCl₃.

Question:

Calculate the formula unit mass of K₂CO₃ given the atomic mass of

Zn = 65 u
Na = 23 u
K = 39 u
C = 12 u
O = 16 u

Concept in a Minute:

Formula unit mass is the sum of the atomic masses of all the atoms in a chemical formula unit. It is calculated by multiplying the atomic mass of each element by the number of atoms of that element in the formula unit and then adding these values.

Explanation:

The chemical formula for potassium carbonate is K2CO3.
This means that one formula unit of potassium carbonate contains:
– 2 atoms of Potassium (K)
– 1 atom of Carbon (C)
– 3 atoms of Oxygen (O)

Given atomic masses:
Atomic mass of K = 39 u
Atomic mass of C = 12 u
Atomic mass of O = 16 u

To calculate the formula unit mass of K2CO3, we sum the atomic masses of all the atoms in the formula unit:

Formula unit mass of K2CO3 = (2 * Atomic mass of K) + (1 * Atomic mass of C) + (3 * Atomic mass of O)
Formula unit mass of K2CO3 = (2 * 39 u) + (1 * 12 u) + (3 * 16 u)
Formula unit mass of K2CO3 = 78 u + 12 u + 48 u
Formula unit mass of K2CO3 = 138 u

Therefore, the formula unit mass of K2CO3 is 138 u.

Question:

Write the chemical formula of the Calcium carbonate.

Concept in a Minute:

Chemical formulas represent the composition of a compound using element symbols and subscript numbers. For ionic compounds, the formula reflects the ratio of ions needed to achieve electrical neutrality. Understanding the charges of common ions is crucial.

Explanation:

Calcium is in Group 2 of the periodic table, so it typically forms a cation with a +2 charge, represented as Ca$^{2+}$.
Carbonate is a polyatomic anion with a formula of CO$_3$ and a charge of -2, represented as CO$_3^{2-}$.
To write the chemical formula for calcium carbonate, we need to find the ratio of Ca$^{2+}$ ions to CO$_3^{2-}$ ions that will result in a neutral compound.
The charge of the calcium ion is +2.
The charge of the carbonate ion is -2.
Since the magnitudes of the charges are equal and opposite (+2 and -2), one calcium ion will combine with one carbonate ion to form a neutral compound.
Therefore, the chemical formula for calcium carbonate is CaCO$_3$.

Question:

Write down the formula of aluminium chloride.

Concept in a Minute:

Chemical formula determination involves understanding the valencies of the constituent elements. Valency is the combining capacity of an element, usually determined by the number of electrons an atom can gain, lose, or share to achieve a stable electron configuration. For ionic compounds, the formula is derived by balancing the charges of the ions.

Explanation:

Aluminium (Al) is in Group 13 of the periodic table and has a valency of +3. This means it tends to lose 3 electrons to form a cation with a charge of Al³⁺.
Chlorine (Cl) is in Group 17 of the periodic table and has a valency of -1. This means it tends to gain 1 electron to form an anion with a charge of Cl⁻.
To form a neutral compound, the total positive charge must balance the total negative charge. We need three chloride ions (each with a -1 charge, total -3) to balance the one aluminium ion (with a +3 charge).
Therefore, the formula for aluminium chloride is AlCl₃.

Question:

Write down the formulae of sodium oxide.

Concept in a Minute:

To write the formula of an ionic compound, you need to know the symbols of the constituent elements and their respective valencies (or charges). The formula is written such that the total positive charge equals the total negative charge, resulting in a neutral compound.

Explanation:

Sodium is an alkali metal, belonging to Group 1 of the periodic table. Elements in Group 1 typically form ions with a +1 charge. Therefore, the symbol for the sodium ion is Na⁺.
Oxygen is a non-metal, belonging to Group 16 of the periodic table. Elements in Group 16 typically form ions with a -2 charge. Therefore, the symbol for the oxide ion is O²⁻.

To form a neutral compound, we need to balance the charges of the sodium ions and the oxide ions. We have a +1 charge from sodium and a -2 charge from oxygen. To balance these, we need two sodium ions (2 x +1 = +2) for every one oxide ion (-2).

Therefore, the formula of sodium oxide is Na₂O.

Question:

Calculate the molar mass of the following substance:

Ethyne, C₂H₂

Concept in a Minute:

Molar mass is the mass of one mole of a substance. It is calculated by summing the atomic masses of all the atoms in a molecule. Atomic mass is the average mass of atoms of an element, measured in atomic mass units (amu) or grams per mole (g/mol).

Explanation:

To calculate the molar mass of ethyne (C2H2), we need to find the atomic masses of carbon (C) and hydrogen (H) and then sum them up according to the molecular formula.
From the periodic table, the atomic mass of carbon (C) is approximately 12.01 g/mol.
The atomic mass of hydrogen (H) is approximately 1.01 g/mol.

In ethyne (C2H2), there are 2 carbon atoms and 2 hydrogen atoms.
Molar mass of C2H2 = (2 × Atomic mass of C) + (2 × Atomic mass of H)
Molar mass of C2H2 = (2 × 12.01 g/mol) + (2 × 1.01 g/mol)
Molar mass of C2H2 = 24.02 g/mol + 2.02 g/mol
Molar mass of C2H2 = 26.04 g/mol

Therefore, the molar mass of ethyne is 26.04 g/mol.

Question:

How many atoms are present in a H₂S molecule?

Concept in a Minute:

A molecule is formed when two or more atoms bond together chemically. The chemical formula of a molecule indicates the type and number of atoms present in one molecule. To find the total number of atoms, we sum up the number of atoms of each element as indicated by the subscripts in the chemical formula.

Explanation:

The chemical formula for hydrogen sulfide is H$_{2}$S.
This formula tells us that one molecule of hydrogen sulfide contains:
– 2 atoms of Hydrogen (indicated by the subscript ‘2’ after ‘H’).
– 1 atom of Sulfur (since there is no subscript after ‘S’, it is understood to be ‘1’).
To find the total number of atoms in a H$_{2}$S molecule, we add the number of hydrogen atoms and the number of sulfur atoms.
Total atoms = (Number of Hydrogen atoms) + (Number of Sulfur atoms)
Total atoms = 2 + 1
Total atoms = 3
Therefore, there are 3 atoms present in a H$_{2}$S molecule.

Question:

Write the chemical formula of the Copper nitrate.

Concept in a Minute:

Ionic compounds are formed from cations and anions. The chemical formula of an ionic compound represents the simplest whole-number ratio of the ions in the compound. To write the formula, we need to know the charges of the cation and anion, and then balance these charges so that the overall compound is neutral.

Explanation:

Copper can exist as different ions, most commonly Cu$^+$ (Copper(I)) and Cu$^{2+}$ (Copper(II)). The question asks for “Copper nitrate”, which typically refers to the more common Copper(II) nitrate.
Nitrate is a polyatomic anion with the formula NO$_3^-$ and a charge of -1.
For Copper(II) nitrate, the copper ion is Cu$^{2+}$ and the nitrate ion is NO$_3^-$.
To balance the charges, we need two nitrate ions (each with a -1 charge) for every one copper ion (with a +2 charge).
Therefore, the chemical formula is Cu(NO$_3$)$_2$.

Question:

Write down the formulae of magnesium hydroxide.

Concept in a Minute:

Chemical formulae represent the elements present in a compound and their respective ratios. This is determined by the valencies of the elements involved. For ionic compounds, the formula is written such that the total positive charge from cations balances the total negative charge from anions.

Explanation:

Magnesium (Mg) is an element from Group 2 of the periodic table, and its common ion is Mg²⁺. Hydroxide is a polyatomic ion with the formula OH⁻. To form a neutral compound, the charges must balance. Since magnesium has a +2 charge and hydroxide has a -1 charge, we need two hydroxide ions to balance one magnesium ion. Therefore, the formula for magnesium hydroxide is Mg(OH)₂. The parentheses around OH are necessary to indicate that the subscript ‘2’ applies to both the oxygen and hydrogen atoms within the hydroxide ion.

Question:

Calculate the formula unit mass of ZnO given the atomic mass of:

Zn = 65 u
Na = 23 u
K = 39 u
C = 12 u
O = 16 u

Concept in a Minute:

Formula unit mass is the sum of the atomic masses of all the atoms present in one formula unit of a compound. For ionic compounds, we use the term “formula unit mass” instead of “molecular mass”.

Explanation:

The question asks to calculate the formula unit mass of ZnO.
We are given the atomic masses of the elements involved:
Atomic mass of Zinc (Zn) = 65 u
Atomic mass of Oxygen (O) = 16 u

In one formula unit of ZnO, there is one atom of Zinc (Zn) and one atom of Oxygen (O).

To calculate the formula unit mass of ZnO, we sum the atomic masses of the constituent elements:
Formula unit mass of ZnO = (Atomic mass of Zn) + (Atomic mass of O)
Formula unit mass of ZnO = 65 u + 16 u
Formula unit mass of ZnO = 81 u

The atomic masses of Na, K, and C are provided but are not relevant to solving this specific question about ZnO.

Question:

Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Concept in a Minute:

The question relies on the Law of Conservation of Mass, specifically the concept of fixed proportions in a chemical compound. Water (H2O) is always formed from hydrogen and oxygen in a fixed mass ratio.

Explanation:

The problem states that hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. This means for every 1 gram of hydrogen, 8 grams of oxygen are required.

We are given that we have 3 grams of hydrogen gas.

Since the ratio is 1:8 (hydrogen:oxygen), we can set up a proportion to find the required mass of oxygen.

If 1 g of hydrogen requires 8 g of oxygen,
then 3 g of hydrogen will require ‘x’ g of oxygen.

1 / 8 = 3 / x

To solve for x, we can cross-multiply:
1 * x = 3 * 8
x = 24

Therefore, 24 grams of oxygen gas would be required to react completely with 3 grams of hydrogen gas.

Question:

Write down the formulae of sodium sulphide.

Concept in a Minute:

Understanding ionic compounds and how to write their chemical formulae. This involves knowing the typical charges of ions formed by elements and balancing those charges to create a neutral compound.

Explanation:

Sodium is an alkali metal (Group 1 of the periodic table), and it typically forms a cation with a +1 charge, denoted as Na⁺. Sulphur is a non-metal from Group 16, and it typically forms an anion with a -2 charge, denoted as S²⁻.

To write the formula for sodium sulphide, we need to ensure that the total positive charge from the sodium ions balances the total negative charge from the sulphide ions, resulting in a neutral compound.

We have Na⁺ and S²⁻.
To balance the charges, we need two sodium ions (2 x +1 = +2) to balance one sulphide ion (-2).

Therefore, the formula for sodium sulphide is Na₂S.

Question:

Give the name of the element present in the following compound:

Baking powder

Concept in a Minute:

Baking powder is a common household ingredient. Understanding its chemical composition is key to identifying the elements present. Baking powder is a mixture of a base (like sodium bicarbonate) and an acid (like tartaric acid or cream of tartar).

Explanation:

Baking powder is a mixture. The primary active ingredient in baking powder is sodium bicarbonate, which has the chemical formula NaHCO3. This compound contains the elements Sodium (Na), Hydrogen (H), Carbon (C), and Oxygen (O). Baking powder also typically contains an acidic salt. A common acidic component is cream of tartar, which is potassium bitartrate (KHC4H4O6). This compound contains Potassium (K), Hydrogen (H), Carbon (C), and Oxygen (O). Therefore, the elements present in baking powder are Sodium, Hydrogen, Carbon, Oxygen, and Potassium.

Question:

What is meant by the term chemical formula?

Concept in a Minute:

Atoms, Elements, Molecules, Compounds, Symbols, Numbers

Explanation:

A chemical formula is a concise way to represent the composition of a chemical compound using the symbols of its constituent elements and numerical subscripts. These subscripts indicate the number of atoms of each element present in one molecule or formula unit of the substance. For example, the chemical formula for water is H₂O. This tells us that one molecule of water consists of two atoms of hydrogen (H) and one atom of oxygen (O). The number ‘1’ is usually omitted in chemical formulas. Similarly, the chemical formula for carbon dioxide is CO₂, indicating one atom of carbon (C) and two atoms of oxygen (O) in one molecule. Chemical formulas are fundamental in chemistry for understanding the structure and reactions of substances.

Question:

Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Concept in a Minute:

Dalton’s atomic theory, Law of definite proportions

Explanation:

Dalton’s atomic theory states that atoms of a given element are identical in mass and properties. It also states that compounds are formed by a combination of atoms of different elements in a fixed ratio. This latter postulate directly explains the law of definite proportions, which states that a chemical compound always contains its component elements in a fixed ratio (by mass) and no matter the source of the compound. For example, water (H₂O) always consists of hydrogen and oxygen in a specific mass ratio, regardless of whether it was obtained from a tap, a river, or synthesized in a laboratory. This fixed ratio is due to the fixed ratio in which the atoms of hydrogen and oxygen combine to form water molecules.

Question:

Calculate the molecular mass of Cl₂.

Concept in a Minute:

To calculate the molecular mass of a compound, you need to sum the atomic masses of all the atoms present in one molecule of the compound. Atomic masses can be found on the periodic table.

Explanation:

The molecule is Cl₂, which means it consists of two chlorine (Cl) atoms.
1. Find the atomic mass of chlorine (Cl) from the periodic table. The atomic mass of chlorine is approximately 35.5 amu (atomic mass units).
2. Since there are two chlorine atoms in a Cl₂ molecule, multiply the atomic mass of chlorine by 2.
Molecular mass of Cl₂ = 2 * (Atomic mass of Cl)
Molecular mass of Cl₂ = 2 * 35.5 amu
Molecular mass of Cl₂ = 71.0 amu

Therefore, the molecular mass of Cl₂ is 71.0 amu.

Question:

Calculate the molecular mass of H₂.

Concept in a Minute:

To calculate the molecular mass of a molecule, you need to sum up the atomic masses of all the atoms present in the molecule. Atomic mass of an element is usually found in the periodic table.

Explanation:

The molecular formula for hydrogen gas is H₂. This means that one molecule of hydrogen is made up of two hydrogen atoms.
First, find the atomic mass of hydrogen from the periodic table. The atomic mass of hydrogen (H) is approximately 1.008 atomic mass units (amu).
Since there are two hydrogen atoms in a molecule of H₂, we multiply the atomic mass of hydrogen by 2.
Molecular mass of H₂ = 2 × (atomic mass of H)
Molecular mass of H₂ = 2 × 1.008 amu
Molecular mass of H₂ = 2.016 amu

Therefore, the molecular mass of H₂ is 2.016 atomic mass units.

Question:

Calculate the molar mass of the following substance:

Hydrochloric acid, HCl

Concept in a Minute:

Molar mass is the mass of one mole of a substance. It is calculated by summing the atomic masses of all the atoms in a molecule, expressed in grams per mole (g/mol). The atomic masses of elements can be found on the periodic table.

Explanation:

To calculate the molar mass of hydrochloric acid (HCl), we need to find the atomic masses of hydrogen (H) and chlorine (Cl) from the periodic table and add them together.

Atomic mass of Hydrogen (H) = approximately 1.008 g/mol
Atomic mass of Chlorine (Cl) = approximately 35.45 g/mol

Molar mass of HCl = Atomic mass of H + Atomic mass of Cl
Molar mass of HCl = 1.008 g/mol + 35.45 g/mol
Molar mass of HCl = 36.458 g/mol

Therefore, the molar mass of hydrochloric acid (HCl) is approximately 36.458 g/mol.

Question:

Calculate the molar mass of the following substance:

Phosphorus molecule, P₄ (atomic mass of phosphorus = 31)

Concept in a Minute:

Molar mass is the mass of one mole of a substance. For a molecule, it is calculated by summing the atomic masses of all the atoms present in the molecule.

Explanation:

The question asks to calculate the molar mass of a phosphorus molecule, which has the chemical formula P₄.
This means that one molecule of phosphorus is composed of 4 phosphorus atoms.
The atomic mass of phosphorus is given as 31.
To find the molar mass of P₄, we need to multiply the atomic mass of phosphorus by the number of phosphorus atoms in the molecule.
Molar mass of P₄ = (Number of P atoms in P₄) × (Atomic mass of P)
Molar mass of P₄ = 4 × 31
Molar mass of P₄ = 124 grams per mole (g/mol)

Therefore, the molar mass of the phosphorus molecule P₄ is 124 g/mol.

Question:

Calculate the molecular mass of C₂H₆.

Concept in a Minute:

Molecular mass is the sum of the atomic masses of all the atoms in a molecule. Atomic mass is found on the periodic table.

Explanation:

To calculate the molecular mass of C2H6, we need to find the atomic masses of Carbon (C) and Hydrogen (H) from the periodic table.
The atomic mass of Carbon (C) is approximately 12.01 u.
The atomic mass of Hydrogen (H) is approximately 1.01 u.

The molecule C2H6 has 2 atoms of Carbon and 6 atoms of Hydrogen.
So, the molecular mass of C2H6 = (2 × Atomic mass of C) + (6 × Atomic mass of H)
Molecular mass of C2H6 = (2 × 12.01 u) + (6 × 1.01 u)
Molecular mass of C2H6 = 24.02 u + 6.06 u
Molecular mass of C2H6 = 30.08 u

Therefore, the molecular mass of C2H6 is 30.08 atomic mass units.

Question:

Write down the name of a compound represented by the following formula:

KNO₃

Concept in a Minute:

This question requires knowledge of chemical nomenclature, specifically how to name ionic compounds based on their chemical formula. The key concepts are identifying the cation and anion from the formula and then recalling or looking up their respective names.

Explanation:

The chemical formula is KNO₃.
This compound is an ionic compound, formed from a cation and an anion.
The first element, K, represents Potassium, which forms the cation K⁺.
The remaining part, NO₃, represents the nitrate polyatomic anion.
Combining the names of the cation and anion, we get Potassium nitrate.

Therefore, the name of the compound represented by the formula KNO₃ is Potassium nitrate.

Question:

Calculate the molecular mass of O₂.

Concept in a Minute:

To calculate the molecular mass of a molecule, you need to sum the atomic masses of all the atoms present in the molecule. The atomic mass of an element is typically found on the periodic table.

Explanation:

The question asks for the molecular mass of O₂.
O₂ is a molecule composed of two oxygen atoms.
To find the molecular mass of O₂, we need the atomic mass of oxygen.
From the periodic table, the atomic mass of oxygen (O) is approximately 16 atomic mass units (amu).
Since there are two oxygen atoms in O₂, the molecular mass of O₂ is calculated as:
Molecular mass of O₂ = (Atomic mass of O) + (Atomic mass of O)
Molecular mass of O₂ = 16 amu + 16 amu
Molecular mass of O₂ = 32 amu

Question:

Write down the name of a compound represented by the following formula:

K₂SO₄

Concept in a Minute:

Ionic compounds are named by combining the name of the cation (positive ion) and the anion (negative ion). The cation is usually a metal, and its name is the same as the metal’s name. The anion is usually a non-metal or a polyatomic ion. If it’s a non-metal, its ending is changed to “-ide”. If it’s a polyatomic ion, its name is used directly.

Explanation:

The formula is K$_{2}$SO$_{4}$.
K represents the potassium ion, which is a cation. The name of the cation is Potassium.
SO$_{4}$ represents the sulfate ion, which is a polyatomic anion. The name of this anion is Sulfate.
Therefore, the compound represented by the formula K$_{2}$SO$_{4}$ is Potassium Sulfate.

Question:

Write down the name of a compound represented by the following formula:

CaCO₃

Concept in a Minute:

Chemical formula interpretation; naming ionic compounds; common polyatomic ions.

Explanation:

The formula CaCO3 represents a compound made of calcium (Ca) and the carbonate ion (CO3). Calcium is an element from Group 2 of the periodic table, and it typically forms a +2 ion (Ca2+). The CO3 group is a polyatomic ion with a charge of -2, known as the carbonate ion. When writing the name of an ionic compound, we combine the name of the cation (the positive ion) with the name of the anion (the negative ion). Therefore, CaCO3 is named calcium carbonate. This is a very common compound found naturally as limestone and is also known as chalk.

Question:

Give the name of the element present in the following compound:

Potassium sulphate

Concept in a Minute:

Chemical compounds are formed from different elements. To identify the elements present in a compound, one needs to know the names of the elements that make up the compound’s name.

Explanation:

The question asks for the name of the elements present in the compound potassium sulphate. The name of the compound itself provides clues to the constituent elements.
“Potassium” directly indicates the presence of the element Potassium (symbol K).
“Sulphate” is a polyatomic ion derived from the element Sulphur (symbol S) and Oxygen (symbol O). Therefore, the elements present in potassium sulphate are Potassium, Sulphur, and Oxygen.

Question:

Why is it not possible to see an atom with naked eyes?

Concept in a Minute:

Size of atoms, limitations of human vision

Explanation:

Atoms are incredibly small, with diameters typically in the range of 0.1 to 0.5 nanometers (nm). A nanometer is one billionth of a meter. To put this into perspective, a human hair is about 80,000 to 100,000 nm wide. Our naked eyes can only resolve objects that are much larger than atoms. The resolving power of the human eye is limited to about 0.1 millimeters (mm) or 100,000 nm. Therefore, because atoms are orders of magnitude smaller than the smallest object we can discern with our eyes, it is impossible to see them without the aid of powerful microscopes.

Question:

Define the atomic mass unit.

Concept in a Minute:

The atomic mass unit (amu) is a standard unit used to express the mass of atoms and molecules. It is defined based on the mass of a specific isotope of carbon.

Explanation:

The atomic mass unit (amu) is defined as one-twelfth (1/12) the mass of an unbound atom of carbon-12 in its ground state. This means that the mass of a carbon-12 atom is taken as exactly 12 amu. This definition provides a convenient and consistent way to compare the masses of different atoms and molecules, as it relates them to a common standard. For practical purposes, 1 amu is approximately equal to 1.660539 x 10^-27 kilograms.

Question:

Calculate the formula unit mass of Na₂O given the atomic mass of

Zn = 65 u
Na = 23 u
K = 39 u
C = 12 u
O = 16 u

Concept in a Minute:

Formula unit mass is the sum of the atomic masses of all atoms in a formula unit of a compound.

Explanation:

To calculate the formula unit mass of Na2O, we need to sum the atomic masses of sodium (Na) and oxygen (O) in the compound.
The given atomic mass of Na is 23 u.
The given atomic mass of O is 16 u.
In the formula unit Na2O, there are 2 atoms of sodium and 1 atom of oxygen.
Therefore, the formula unit mass of Na2O is (2 × atomic mass of Na) + (1 × atomic mass of O).
Formula unit mass of Na2O = (2 × 23 u) + (1 × 16 u)
Formula unit mass of Na2O = 46 u + 16 u
Formula unit mass of Na2O = 62 u.

Question:

Give the name of the element present in the following compound:

Hydrogen bromide

Concept in a Minute:

Chemical nomenclature involves naming compounds based on the elements they contain. For simple binary compounds, the name often directly indicates the constituent elements.

Explanation:

The compound is named “Hydrogen bromide”. The name explicitly states the two elements present. The first part, “Hydrogen”, refers to the element Hydrogen. The second part, “bromide”, indicates the presence of the element Bromine. Therefore, the elements present in hydrogen bromide are Hydrogen and Bromine.

Question:

Give the name of the element present in the following compound:

Quick lime

Concept in a Minute:

Chemical names of common compounds and their constituent elements. Understanding that common names are often used for well-known chemical substances.

Explanation:

Quicklime is the common name for the chemical compound calcium oxide. Calcium oxide has the chemical formula CaO. The elements present in calcium oxide are calcium (Ca) and oxygen (O). Therefore, the elements present in quicklime are calcium and oxygen.

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