Question 1: 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 of water, and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.
Answer:
Total mass of reactants:
- Mass of sodium carbonate = 5.3 g
- Mass of acetic acid = 6.0 g
- Total mass of reactants = 5.3 g + 6.0 g = 11.3 g
Total mass of products:
- Mass of carbon dioxide = 2.2 g
- Mass of water = 0.9 g
- Mass of sodium acetate = 8.2 g
- Total mass of products = 2.2 g + 0.9 g + 8.2 g = 11.3 g
Since the total mass of reactants equals the total mass of products (11.3 g), the observations agree with the law of conservation of mass.
Question 2: 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?
Answer:
The ratio of hydrogen to oxygen by mass is 1:8.
For 1 g of hydrogen, oxygen required = 8 g.
For 3 g of hydrogen:
- Oxygen required = 3 g × 8 = 24 g
Therefore, 24 g of oxygen gas is required to react completely with 3 g of hydrogen gas.
Question 3: Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Answer:
The postulate stating that atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction, is a result of the law of conservation of mass.
Question 4: Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Answer:
The postulate stating that atoms of different elements combine in the ratio of small whole numbers to form compounds explains the law of definite proportions.
Question 5: Define the atomic mass unit.
Answer:
An atomic mass unit (u) is defined as one-twelfth (1⁄12) of the mass of one atom of carbon-12 isotope.
Question 6: Why is it not possible to see an atom with naked eyes?
Answer:
Atoms are extremely small, with sizes around 10⁻¹⁰ meters. They are too tiny to be seen with the naked eye.
Question 7: Write down the formulae of:
(i) Sodium oxide
(ii) Aluminium chloride
(iii) Sodium sulphide
(iv) Magnesium hydroxide
Answer:
(i) Sodium oxide: Na₂O
(ii) Aluminium chloride: AlCl₃
(iii) Sodium sulphide: Na₂S
(iv) Magnesium hydroxide: Mg(OH)₂
Question 8: Write down the names of compounds represented by the following formulae:
(i) Al₂(SO₄)₃
(ii) CaCl₂
(iii) K₂SO₄
(iv) KNO₃
(v) CaCO₃
Answer:
(i) Aluminium sulphate
(ii) Calcium chloride
(iii) Potassium sulphate
(iv) Potassium nitrate
(v) Calcium carbonate
Question 9: What is meant by the term chemical formula?
Answer:
A chemical formula is a symbolic representation of a substance, showing the elements present and the number of atoms of each element in one molecule of the substance.
Question 10: How many atoms are present in:
(i) An H₂S molecule
(ii) A PO₄³⁻ ion
Answer:
(i) H₂S molecule: 2 hydrogen atoms + 1 sulfur atom = 3 atoms
(ii) PO₄³⁻ ion: 1 phosphorus atom + 4 oxygen atoms = 5 atoms
Question 11: Calculate the molecular masses of:
H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH.
Answer:
Atomic masses: H = 1 u, C = 12 u, O = 16 u, N = 14 u, Cl = 35.5 u.
(i) H₂: 2 × 1 = 2 u
(ii) O₂: 2 × 16 = 32 u
(iii) Cl₂: 2 × 35.5 = 71 u
(iv) CO₂: 12 + (2 × 16) = 12 + 32 = 44 u
(v) CH₄: 12 + (4 × 1) = 12 + 4 = 16 u
(vi) C₂H₆: (2 × 12) + (6 × 1) = 24 + 6 = 30 u
(vii) C₂H₄: (2 × 12) + (4 × 1) = 24 + 4 = 28 u
(viii) NH₃: 14 + (3 × 1) = 14 + 3 = 17 u
(ix) CH₃OH: 12 + (4 × 1) + 16 = 12 + 4 + 16 = 32 u
Question 12: Calculate the formula unit masses of ZnO, Na₂O, K₂CO₃, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
Answer:
(i) ZnO: 65 + 16 = 81 u
(ii) Na₂O: (2 × 23) + 16 = 46 + 16 = 62 u
(iii) K₂CO₃: (2 × 39) + 12 + (3 × 16) = 78 + 12 + 48 = 138 u
Question 13: A 0.24 g sample of a 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:
Total mass of compound = 0.24 g
- Percentage of boron = (0.096 g / 0.24 g) × 100% = 40%
- Percentage of oxygen = (0.144 g / 0.24 g) × 100% = 60%
Therefore, the compound is 40% boron and 60% oxygen by weight.
Question 14: When 3.0 g of carbon is burnt in 8.00 g of 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:
Carbon burns in oxygen to form carbon dioxide:
- C + O₂ → CO₂
From the given data:
- 3.0 g C + 8.0 g O₂ → 11.0 g CO₂
When 3.0 g of carbon is burnt in 50.00 g of oxygen, carbon is the limiting reagent and will react with only 8.0 g of oxygen.
- Excess oxygen = 50.00 g – 8.0 g = 42.0 g (unreacted)
Thus, the mass of carbon dioxide formed will still be 11.0 g.
The law of definite proportions governs this answer, stating that a chemical compound always contains the same elements in the exact same proportions by mass.
Question 15: What are polyatomic ions? Give examples.
Answer:
Polyatomic ions are ions that consist of a group of atoms bonded together and carry an overall charge. Examples include:
- Ammonium ion: NH₄⁺
- Hydroxide ion: OH⁻
- Sulphate ion: SO₄²⁻
- Nitrate ion: NO₃⁻
Question 16: Write the chemical formulae of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate
Answer:
(a) Magnesium chloride: MgCl₂
(b) Calcium oxide: CaO
(c) Copper nitrate: Cu(NO₃)₂
(d) Aluminium chloride: AlCl₃
(e) Calcium carbonate: CaCO₃
Question 17: Give the names of the elements present in the following compounds:
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate
Answer:
(a) Quick lime: Calcium (Ca) and Oxygen (O)
(b) Hydrogen bromide: Hydrogen (H) and Bromine (Br)
(c) Baking powder: Sodium (Na), Hydrogen (H), Carbon (C), and Oxygen (O)
(d) Potassium sulphate: Potassium (K), Sulphur (S), and Oxygen (O)
Question 18: Calculate 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:
(a) Ethyne (C₂H₂):
- Molar mass = (2 × 12) + (2 × 1) = 24 + 2 = 26 g/mol
(b) Sulphur molecule (S₈):
- Molar mass = 8 × 32 = 256 g/mol (Atomic mass of S = 32 u)
(c) Phosphorus molecule (P₄):
- Molar mass = 4 × 31 = 124 g/mol
(d) Hydrochloric acid (HCl):
- Molar mass = 1 + 35.5 = 36.5 g/mol
(e) Nitric acid (HNO₃):
- Molar mass = 1 + 14 + (3 × 16) = 1 + 14 + 48 = 63 g/mol