Question 1

Gay-Lussac’s law of gaseous volumes could NOT be explained by Dalton’s atomic theory. It was satisfactorily explained by:

Accepted Answer

Avogadro’s law. Dalton believed atoms of the same element could not combine, so his theory failed to explain why gases react in simple volume ratios. Avogadro’s law (equal volumes contain equal numbers of molecules), together with the idea of diatomic molecules such as H₂ and O₂, explained Gay-Lussac’s observations.

Question 2

Which of the following pairs of compounds can be used to illustrate the law of multiple proportions?

Accepted Answer

SO₂ and SO₃. The law of multiple proportions applies when the SAME two elements form more than one compound. SO₂ and SO₃ are both made of only sulphur and oxygen (for a fixed 32 g of S, oxygen is 32 g and 48 g, ratio 2 : 3). The other pairs do not consist of the same two elements.

Question 3

Two oxides of nitrogen are analysed. Oxide A contains 63.6% nitrogen and oxide B contains 46.7% nitrogen by mass. For a fixed mass of nitrogen, the ratio of the mass of oxygen in A to that in B is:

Accepted Answer

1 : 2. In A, per 63.6 g N there is 36.4 g O, so O per gram of N = 36.4 ÷ 63.6 = 0.572. In B, per 46.7 g N there is 53.3 g O, so O per gram of N = 53.3 ÷ 46.7 = 1.143. Ratio = 0.572 : 1.143 = 1 : 2 (A is N₂O, B is NO), confirming the law of multiple proportions.

Question 4

10 mL of methane is burnt in excess oxygen at 120 °C: CH₄ + 2O₂ → CO₂ + 2H₂O. With all gases measured at the same temperature and pressure, the volume of oxygen consumed and the total volume of gaseous products are respectively:

Accepted Answer

20 mL and 30 mL. By Gay-Lussac’s law volumes follow the coefficients. For 10 mL CH₄: oxygen = 2 × 10 = 20 mL. Products = CO₂ (10 mL) + H₂O vapour (2 × 10 = 20 mL) = 30 mL. Forgetting the water vapour (counting only 10 mL CO₂) gives the wrong 10 mL for products.

Question 5

12.25 g of potassium chlorate is heated and decomposes completely into potassium chloride and oxygen gas, leaving 7.45 g of potassium chloride. By the law of conservation of mass, the mass of oxygen gas released is:

Accepted Answer

4.8 g. 2KClO₃ → 2KCl + 3O₂. Mass of oxygen = mass of reactant − mass of solid product = 12.25 − 7.45 = 4.8 g. Adding the masses (12.25 + 7.45) wrongly gives 19.7 g.

Question 6

A 2.2 g sample of pure carbon dioxide obtained from burning coal contains 0.6 g of carbon. By the law of definite proportions, the mass of carbon present in a 4.4 g sample of pure carbon dioxide obtained by fermentation is:

Accepted Answer

1.2 g. Pure CO₂ always has the same composition, so the carbon fraction is 0.6 ÷ 2.2 = 27.3% regardless of source. In 4.4 g (twice the mass) the carbon is simply doubled: 2 × 0.6 = 1.2 g.

Question 7

At the same temperature and pressure, 1 L of gas X contains 2.0 × 10²² molecules. Under these identical conditions, the number of molecules present in 2.5 L of a different gas Y is:

Accepted Answer

5.0 × 10²². By Avogadro’s law, at the same T and P the number of molecules depends only on the volume, not on the identity of the gas. Number ∝ volume, so 2.5 L contains 2.5 × (2.0 × 10²²) = 5.0 × 10²² molecules.

Laws of Chemical Combination — JEE Main Chemistry MCQs with Solutions

Questions with solutions