1 Mark 2020
State the Law of Conservation of Mass.
Ans: In a chemical reaction, the total mass of reactants equals the total mass of products. Mass is neither created nor destroyed. (Antoine Lavoisier, 1789)
2 Mark 2019
What is Avogadro's number? Calculate the number of molecules in 22 g of CO₂.
Ans: Avogadro's number = 6.022 × 10²³. It is the number of atoms/molecules/ions in one mole of a substance. Molar mass of CO₂ = 44 g/mol. Moles of CO₂ = 22/44 = 0.5 mol. Number of molecules = 0.5 × 6.022 × 10²³ = 3.011 × 10²³ molecules.
2 Mark 2018
Write the chemical formulae of: (a) Ammonium sulphate, (b) Aluminium oxide, (c) Calcium carbonate, (d) Sodium nitrate.
Ans: (a) Ammonium sulphate: (NH₄)₂SO₄ [NH₄⁺ and SO₄²⁻]. (b) Aluminium oxide: Al₂O₃ [Al³⁺ and O²⁻]. (c) Calcium carbonate: CaCO₃ [Ca²⁺ and CO₃²⁻]. (d) Sodium nitrate: NaNO₃ [Na⁺ and NO₃⁻].
3 Mark 2022
Calculate the molar mass of: (a) H₂SO₄, (b) Ca(OH)₂, (c) C₆H₁₂O₆. Also find the number of moles in 49 g of H₂SO₄.
Ans: (a) H₂SO₄: 2(1) + 32 + 4(16) = 2 + 32 + 64 = 98 g/mol. (b) Ca(OH)₂: 40 + 2(16+1) = 40 + 34 = 74 g/mol. (c) C₆H₁₂O₆: 6(12) + 12(1) + 6(16) = 72 + 12 + 96 = 180 g/mol. Moles of H₂SO₄ = 49/98 = 0.5 mol.
1 Mark 2017
What is atomicity? Give the atomicity of ozone and phosphorus.
Ans: Atomicity is the number of atoms present in one molecule of an element. Ozone (O₃): atomicity = 3. Phosphorus (P₄): atomicity = 4.
1 Mark CBSE 2019
What is the value of Avogadro's number?
Ans: Avogadro's number is 6.022 × 10²³. It is the number of particles in one mole of any substance.
1 Mark CBSE 2021
Who proposed the atomic theory of matter in 1808?
Ans: John Dalton proposed the atomic theory in 1808.
2 Mark CBSE 2020
Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, NH₃ and CH₃OH. (H = 1, C = 12, N = 14, O = 16, Cl = 35.5)
Ans: H₂ = 2 u; O₂ = 32 u; Cl₂ = 71 u; CO₂ = 12 + 32 = 44 u; CH₄ = 12 + 4 = 16 u; NH₃ = 14 + 3 = 17 u; CH₃OH = 12 + 3 + 16 + 1 = 32 u.
2 Mark CBSE 2022
Calculate the formula unit masses of ZnO, Na₂O and K₂CO₃. (Zn = 65, Na = 23, K = 39, C = 12, O = 16)
Ans: ZnO = 65 + 16 = 81 u. Na₂O = (2 × 23) + 16 = 62 u. K₂CO₃ = (2 × 39) + 12 + (3 × 16) = 78 + 12 + 48 = 138 u.
2 Mark CBSE 2018
If one mole of carbon atoms weighs 12 g, what is the mass of one atom of carbon?
Ans: One mole contains 6.022 × 10²³ atoms. Mass of one atom = 12 ÷ (6.022 × 10²³) = 1.99 × 10⁻²³ g.
3 Mark CBSE 2023
Which has more atoms: 100 g of sodium or 100 g of iron? (Na = 23 u, Fe = 56 u)
Ans: Moles of Na = 100 ÷ 23 = 4.35 mol. Moles of Fe = 100 ÷ 56 = 1.79 mol. Since 1 mole of any element has the same number of atoms (6.022 × 10²³), 4.35 mol of sodium has more atoms than 1.79 mol of iron. So 100 g of sodium has more atoms.
3 Mark CBSE 2020
A 0.24 g sample of a compound of oxygen and boron contains 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by mass.
Ans: Mass percentage of boron = (0.096 ÷ 0.24) × 100 = 40%. Mass percentage of oxygen = (0.144 ÷ 0.24) × 100 = 60%. Check: 40% + 60% = 100%.
3 Mark CBSE 2024
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 governs your answer?
Ans: 3.00 g of carbon combines with exactly 8.00 g of oxygen to form 11.00 g of CO₂. In 50.00 g of oxygen, only 8.00 g reacts and the remaining oxygen is left unused. So 11.00 g of carbon dioxide is formed. This illustrates the Law of Constant Proportions, as carbon and oxygen always combine in a fixed mass ratio of 3 : 8.
5 Mark CBSE 2022
Calculate the number of moles in: (a) 12 g of oxygen gas, (b) 20 g of water, (c) 22 g of carbon dioxide. (O = 16, H = 1, C = 12)
Ans: (a) Molar mass of O₂ = 32 g/mol, so moles = 12 ÷ 32 = 0.375 mol. (b) Molar mass of H₂O = 18 g/mol, so moles = 20 ÷ 18 = 1.11 mol. (c) Molar mass of CO₂ = 44 g/mol, so moles = 22 ÷ 44 = 0.5 mol. Formula used: moles = given mass ÷ molar mass.
5 Mark CBSE 2021
Explain the criss-cross method of writing chemical formulae. Write the formulae of sodium oxide, aluminium chloride, sodium sulphide and magnesium hydroxide.
Ans: Criss-cross method: write the symbols with their valencies, cross over the valencies so that each becomes the subscript of the other element, and reduce to the simplest ratio. Sodium oxide: Na⁺ and O²⁻ give Na₂O. Aluminium chloride: Al³⁺ and Cl⁻ give AlCl₃. Sodium sulphide: Na⁺ and S²⁻ give Na₂S. Magnesium hydroxide: Mg²⁺ and OH⁻ give Mg(OH)₂, with the hydroxide group in brackets because two are needed.
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