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NCERT Solutions for Class 9th Science Chapter 9 .2 Law of Constant Proportions — Pause and Ponder

Book page 1679 Updated on2026-09-08

Q3.
A compound consists of 40% sulfur and 60% oxygen by mass. In a sample of the same compound containing 20 g of sulfur, what mass of oxygen must be present to satisfy the Law of Constant Proportions?
Answer

30 g of oxygen.

In the compound, S : O = 40 : 60 = 2 : 3 by mass
So, mass of oxygen = mass of sulfur × 3 ÷ 2
= 20 g × 3 ÷ 2
= 30 g

Check: total mass of sample = 20 g + 30 g = 50 g. Percentage of sulfur = 20 ÷ 50 × 100 = 40 %, and of oxygen = 30 ÷ 50 × 100 = 60 % ✓

Why it happens: the Law of Constant Proportions says a compound has a fixed composition by mass no matter how big the sample or where it came from. The percentages therefore stay 40 % and 60 % whether you have 5 g of the compound or 5 kg, so the S : O mass ratio 2 : 3 must hold in every sample.
Did you know? With S = 32 u and O = 16 u, the ratio 20 g : 30 g means 20/32 : 30/16 = 0.625 : 1.875 = 1 : 3 by number of atoms — so the compound is SO₃, sulfur trioxide.
Q4.
Carbon monoxide (CO) contains carbon and oxygen in the mass ratio of 3:4. How much oxygen will combine with 9 g of carbon to form carbon monoxide?
Answer

12 g of oxygen.

C : O = 3 : 4 by mass
Mass of oxygen = mass of carbon × 4 ÷ 3
= 9 g × 4 ÷ 3
= 12 g

Check: 9 g : 12 g = 3 : 4 ✓. Total mass of carbon monoxide formed = 9 g + 12 g = 21 g, by the Law of Conservation of Mass.

Why it happens: the ratio 3 : 4 is just the atomic masses in disguise. One CO molecule has one C atom (12 u) and one O atom (16 u), and 12 : 16 = 3 : 4. Because every molecule of the compound is built the same way, the mass ratio in any quantity of it is the same 3 : 4.
Tip: if you were asked for carbon dioxide instead, the ratio would be 12 : 32 = 3 : 8, and 9 g of carbon would need 24 g of oxygen. Same two elements, different compound, different fixed ratio — the law is about one compound at a time.
Q5.
The Law of Definite Proportions holds true for compounds but not for mixtures. Give reason.
Answer

Because a compound is held together by chemical bonds in a fixed pattern of atoms, while a mixture is only a physical jumble whose parts can be present in any amount.

CompoundMixture
How the parts are heldBy chemical bonds — a fixed number of atoms of each element per molecule or formula unitNot held at all; the components simply lie together
Composition by massFixed — water is always 1 : 8 H : OAny value you like
ExampleH₂O; 9 g always gives 1 g H and 8 g OA mixture of H₂ and O₂ gases, or salt in water, in any ratio
PropertiesNew properties, quite unlike those of the elementsEach component keeps its own properties

In water, two hydrogen atoms are bonded to one oxygen atom, always. That count cannot be changed without destroying the substance; change it and you no longer have water. So the mass ratio is locked at 2 × 1 u : 1 × 16 u = 1 : 8.

A mixture has no such lock. You can stir 1 g of salt into a litre of water, or 30 g, and both are still salt solution.

Why it happens: the fixed ratio comes from Dalton's postulate that atoms combine in the ratio of simple whole numbers. A whole number cannot slide gradually — there is no molecule of water with 2.3 hydrogen atoms. Mixing involves no atoms combining at all, so nothing constrains the proportions.
Q6.
Students X and Y, both prepared an oxide of copper by combining copper and oxygen in the ratios of 4:1 and 8:2, respectively. Do their results justify the Law of Constant Proportions? Explain.
Answer

Yes, their results justify the law — because 8 : 2 is the same ratio as 4 : 1.

Student X: Cu : O = 4 : 1
Student Y: Cu : O = 8 : 2 = (8 ÷ 2) : (2 ÷ 2) = 4 : 1
Both ratios are identical.

Compare them as percentages, which is the fairest test:

StudentCu : O as taken% copper% oxygen
X4 : 14/5 × 100 = 80 %1/5 × 100 = 20 %
Y8 : 28/10 × 100 = 80 %2/10 × 100 = 20 %

Same composition by mass, from two independent preparations — which is exactly what the Law of Constant Proportions predicts. Student Y has simply made twice as much of the same oxide.

Why it happens: a ratio measures proportion, not amount. Doubling both numbers doubles the size of the sample but leaves the composition untouched, just as 2 chapatis for 1 bowl of dal is the same recipe as 4 chapatis for 2 bowls.
Did you know? With Cu = 63.5 u and O = 16 u, a 4 : 1 mass ratio means 4/63.5 : 1/16 ≈ 1 : 1 by number of atoms — so both students have made CuO, black copper(II) oxide.
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