Class 9 – Science Extra Questions – Ch. 3 – Atoms and Molecules

Ans: C

Explanation: The chemical symbol Au is derived from the Latin word for gold, which is “aurum”. Therefore, the element with the chemical symbol Au is gold. Aluminum has the symbol Al, Argon has the symbol Ar, and Silver has the symbol Ag.
Correct Option: C

Ans: C

Explanation: An atom is defined as the smallest unit of an element that retains the chemical properties of that element. While atoms are incredibly small, the concept of “smallest particle of matter known” is not entirely accurate as subatomic particles (protons, neutrons, electrons) are smaller. Option B is too restrictive as atoms are fundamental to all states of matter, not just gases. Option D describes a type of radiation, not an atom. Option C correctly identifies that an atom is the smallest particle of an element that can participate in a chemical reaction, which is a key characteristic of atoms.
Correct Option: C

Ans: D

Explanation: Calcium is in Group 2 of the periodic table and forms a +2 ion (Ca²⁺). The carbonate ion is a polyatomic ion with the formula CO₃²⁻ and a charge of -2. To form a neutral compound, the charges must balance. One calcium ion (Ca²⁺) will combine with one carbonate ion (CO₃²⁻) because the charges are equal and opposite (+2 and -2). Therefore, the chemical formula for calcium carbonate is CaCO₃. Option A has an incorrect calcium to carbonate ratio. Option B is not a valid chemical formula. Option C suggests two carbonate ions for one calcium ion, which would result in a net negative charge.
Correct Option: D

Ans: D

Explanation: The total weight of the compound is 10 g. The weight of element X in the compound is 4 g. The weight of element Y in the compound is the total weight minus the weight of element X, which is 10 g – 4 g = 6 g. The percentage composition of element Y by weight is calculated by dividing the weight of element Y by the total weight of the compound and multiplying by 100. So, the percentage composition of element Y is (6 g / 10 g) * 100 = 60%.
Correct Option: D

Ans: C

Explanation: To determine the number of atoms in a given mass of a substance, we first need to find the number of moles of that substance. The atomic mass of Helium (He) is approximately 4 grams per mole. Therefore, 52 grams of Helium would be 52 grams / 4 grams/mole = 13 moles. One mole of any substance contains Avogadro’s number of particles, which is approximately 6.022 × 10^23. So, the number of atoms in 13 moles of Helium is 13 moles × 6.022 × 10^23 atoms/mole = 13 × 6.022 × 10^23 atoms.
Correct Option: C

Ans: C

Explanation: Sodium chloride is an ionic compound with the chemical formula NaCl. This formula indicates that it is composed of sodium (Na) and chlorine (Cl) atoms. Therefore, sodium and chlorine are the elements present in sodium chloride. Carbon (C) is a different element and is not a component of sodium chloride. Option D is incorrect because only carbon is not present, while chlorine is present.
Correct Option: C

Ans: A

Explanation: Sodium (Na) is in Group 1 of the periodic table and typically forms a +1 ion (Na+). Chlorine (Cl) is in Group 17 and typically forms a -1 ion (Cl-). To form a neutral ionic compound, one sodium ion (Na+) must combine with one chloride ion (Cl-). Therefore, the chemical formula is NaCl. Option B is incorrect because it implies two sodium atoms for every one chlorine atom, which would result in a net positive charge. Option C is incorrect as it suggests a +1 sodium ion and a -2 chloride ion, which is not how chlorine typically bonds. Option D suggests two sodium ions for every two chloride ions, which simplifies to the correct 1:1 ratio but is not the standard representation of the empirical formula.
Correct Option: A

Ans: B

Explanation: A neutral atom has an equal number of protons and electrons. A cation is a positively charged ion, meaning it has more protons than electrons. An anion is a negatively charged ion, meaning it has more electrons than protons. A molecule is a group of two or more atoms held together by chemical bonds. A compound is a substance formed when two or more chemical elements are chemically bonded together. In particle X, the number of protons (17) is less than the number of electrons (18). This means particle X has a net negative charge and is therefore an anion.
Correct Option: B

Ans: C

Explanation: The chemical formula for glucose is C6H12O6. This formula indicates that each molecule of glucose contains 6 atoms of carbon (C), 12 atoms of hydrogen (H), and 6 atoms of oxygen (O). To find the total number of atoms in a single molecule, we sum the number of atoms of each element: 6 (carbon) + 12 (hydrogen) + 6 (oxygen) = 24 atoms.
Correct Option: C

Ans: A

Explanation: The molar mass of cinnabar (HgS) is the sum of the molar masses of Hg and S. Molar mass of HgS = Molar mass of Hg + Molar mass of S = 200.6 g/mol + 32 g/mol = 232.6 g/mol.
The mass percentage of sulfur in HgS is calculated as (Molar mass of S / Molar mass of HgS) * 100%.
Mass percentage of S = (32 g/mol / 232.6 g/mol) * 100% = 0.137575 * 100% = 13.7575%.
Rounding to one decimal place, this is 13.8%. Looking at the options, option A is the closest.

Let’s recheck the calculation with the provided options.
If A is 13.7%, then (32 / (200.6 + 32)) * 100 = (32 / 232.6) * 100 = 13.7575…%, which rounds to 13.8%.
However, if we consider the options provided, 13.7% is the closest. Let’s assume there might be a slight rounding difference in the provided molar masses or the options.

Let’s calculate the percentage for sulfur using the given molar masses:
Molar mass of Hg = 200.6 g/mol
Molar mass of S = 32 g/mol
Molar mass of HgS = 200.6 + 32 = 232.6 g/mol

Mass percentage of S = (Mass of S / Molar mass of HgS) * 100
Mass percentage of S = (32 / 232.6) * 100
Mass percentage of S = 13.7575… %

Comparing this to the options:
A. 13.7%
B. 15.9%
C. 84.1%
D. 86.3%

13.7575% is closest to 13.7%.

Let’s check if using rounded values for molar masses gives a closer result. For instance, if we approximate S to 32.06 g/mol (common value), then Molar mass of HgS = 200.6 + 32.06 = 232.66 g/mol.
Mass percentage of S = (32.06 / 232.66) * 100 = 13.779… %
This is still closest to 13.7%.

If we consider Hg’s molar mass more precisely, e.g., 200.59 g/mol, and S as 32.065 g/mol:
Molar mass of HgS = 200.59 + 32.065 = 232.655 g/mol
Mass percentage of S = (32.065 / 232.655) * 100 = 13.782… %
Again, closest to 13.7%.

Let’s assume the question intends for us to use the given molar masses strictly.
Molar mass of HgS = 200.6 + 32 = 232.6 g/mol
Mass percentage of sulfur = (32 / 232.6) * 100 = 13.7575…%

Option A is 13.7%. The difference is 0.0575%.
Let’s calculate the mass percentage of Hg:
Mass percentage of Hg = (200.6 / 232.6) * 100 = 86.2425…%
This is close to option D (86.3%).
If the mass percentage of Hg is approximately 86.3%, then the mass percentage of S would be 100% – 86.3% = 13.7%.
This confirms that option A is likely the correct answer based on the provided data.

Let’s consider option B: 15.9%. This is significantly different.
Let’s consider option C: 84.1%. This would imply sulfur is 100 – 84.1 = 15.9%. This is option B, so C is incorrect if B is the % of Hg.

The calculation: (32 / 232.6) * 100 = 13.7575…%
This is very close to 13.7%.

Let’s assume there might be a rounding in the options.
If option A is 13.7%, and our calculation is 13.7575%, the difference is 0.0575%.
If we had options like 13.76%, it would be more precise. Given the options, 13.7% is the best fit.

Final check:
Mass of Hg = 200.6
Mass of S = 32
Total mass = 232.6
%S = (32 / 232.6) * 100 = 13.7575…
This value is closest to 13.7%.

Correct Option: A
Correct Option: A