Chapter 3: Atoms And Molecules Exercise Questions & Answers
1A 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.
Answer & Reference:
% of boron = (0.096 / 0.24) × 100 = 40%
% of oxygen = (0.144 / 0.24) × 100 = 60%
(Ref: Page 36, Exercise 1; concept from Page 17, Example 2.1)
2When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?
Answer & Reference:
Only 8 g oxygen reacts with 3 g carbon → 11 g CO₂ is produced (excess oxygen remains).
This follows the Law of Constant Proportions.
(Ref: Page 36, Exercise 2; Page 27, Section 3.1.2)
3What are polyatomic ions? Give examples.
Answer & Reference: Polyatomic ions are groups of atoms with a net charge.
Examples: NH₄⁺ (ammonium), SO₄²⁻ (sulphate), NO₃⁻ (nitrate), OH⁻ (hydroxide).
(Ref: Page 32, Section 3.3.3; Table 3.6)
4Write the chemical formulae of the following. (a) Magnesium chloride (b) Calcium oxide (c) Copper nitrate (d) Aluminium chloride (e) Calcium carbonate
5Give the names of the elements present in the following compounds. (a) Quick lime (b) Hydrogen bromide (c) Baking powder (d) Potassium sulphate
Need a refresher on **Compounds**? Review the
Classification of Matter (Is Matter Around Us Pure?)
before you begin calculations.
Answer & Reference:
(a) Quick lime (CaO): Calcium, Oxygen
(b) Hydrogen bromide (HBr): Hydrogen, Bromine
(c) Baking powder (NaHCO₃): Sodium, Hydrogen, Carbon, Oxygen
(d) Potassium sulphate (K₂SO₄): Potassium, Sulphur, Oxygen
(Ref: Page 36, Exercise 5; Table 3.1 for element symbols)
6Calculate the molar mass of the following substances. (a) Ethyne, C₂H₂ (b) Sulphur molecule, S₈ (c) Phosphorus molecule, P₄ (Atomic mass of phosphorus = 31) (d) Hydrochloric acid, HCl (e) Nitric acid, HNO₃
Answer & Reference:
(a) C₂H₂ = 2×12 + 2×1 = 26 u
(b) S₈ = 8×32 = 256 u
(c) P₄ = 4×31 = 124 u
(d) HCl = 1 + 35.5 = 36.5 u
(e) HNO₃ = 1 + 14 + 3×16 = 63 u
(Ref: Page 36, Exercise 6; Page 34, Section 3.5; Table 3.2 for atomic masses)