Q7.
Assertion (A): 2 g of hydrogen combines with 16 g of oxygen to form 18 g of water. Reason (R): According to Dalton’s Atomic Theory, atoms combine in a simple whole number ratio by mass to form compounds. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.
Answer
Option (ii) — Both A and R are true, but R is not the correct explanation of A.
Is A true? Yes, on two counts.
Ratio taken, H : O = 2 g : 16 g = 1 : 8 — the fixed ratio in water ✓
Mass of water = 2 g + 16 g = 18 g — by conservation of mass ✓
(Check with the formula: H₂O = (1 u × 2) + 16 u = 18 u, and H : O = 2 : 16 = 1 : 8)
Mass of water = 2 g + 16 g = 18 g — by conservation of mass ✓
(Check with the formula: H₂O = (1 u × 2) + 16 u = 18 u, and H : O = 2 : 16 = 1 : 8)
Is R true? It is a statement of Dalton's fifth postulate — atoms combine in the ratio of simple whole numbers to form compounds — and it is a correct principle.
Does R explain A? No. A makes two claims, and Dalton's whole-number postulate delivers neither of them:
- That the mass ratio is 1 : 8 comes from Proust's Law of Constant Proportions, not from Dalton's whole-number rule.
- That 2 g + 16 g gives exactly 18 g comes from the Law of Conservation of Mass.
Dalton's postulate tells you how many atoms join (2 H to 1 O). It does not by itself tell you what masses will be used up or produced. So R is a true statement placed beside a true statement — but it is not the reason for it.
Tip: read R strictly and it has one more flaw — Dalton's postulate is about the ratio of the number of atoms, not of masses. A student who marks (iii) on that reading has spotted something real; but since 1 : 8 does happen to be a simple whole-number mass ratio here, R is best treated as true and the deciding point is that it does not explain A. Hence (ii).