Q.A sample of pure water, whatever the source always contains ......... by mass of oxygen and 11.1 % by mass of hydrogen. a. 88.9 b. 18 c. 80 d. 16
Concept understanding — Law of Definite Proportions
The Law of Definite Proportions
Imagine you are making lemonade. You decide the perfect glass has 2 spoons of sugar, the juice of 1 lemon, and 200 ml of water. If you make it exactly this way every time, the taste is identical. But if you double the water or halve the sugar, you get a different drink — not the same lemonade anymore.
A chemical compound works the same way. Water is not just "hydrogen and oxygen" — it is hydrogen and oxygen in a very specific, unchangeable mass ratio. If you change that ratio, you no longer have water. You might get hydrogen peroxide, or just a mixture of gases, but not the same compound.
The law applies to compounds (pure substances made of different elements bonded together), not to mixtures. In a mixture, you can vary the proportions freely — think of a handful of sand and salt.
The Precise Statement
A given chemical compound always contains its constituent elements in a fixed ratio by mass, regardless of its source or method of preparation.
This means: every molecule of water, whether it comes from rain, a river, your tap, or is synthesized in a lab, has exactly the same mass ratio of hydrogen to oxygen.
The Numbers That Never Change
Take water (H2O). In every single water molecule:
- 2 hydrogen atoms (atomic mass ≈ 1 u each) → total mass = 2 u
- 1 oxygen atom (atomic mass ≈ 16 u) → total mass = 16 u
The mass ratio of hydrogen to oxygen is always 2:16, which simplifies to 1:8.
mass of oxygenmass of hydrogen=162=81
If you decompose 18 grams of water, you will always get exactly 2 grams of hydrogen and 16 grams of oxygen. Not 2.5 g and 15.5 g — always 2 g and 16 g.
Why This Matters
This law was a cornerstone for John Dalton's atomic theory. It told scientists that elements combine in fixed, whole-number ratios because atoms themselves are indivisible units that combine in fixed numbers. If you could vary the ratio, atoms would have to be splittable — which they aren't, in chemical reactions.
A common mistake is to think the law says "the ratio of elements is the same in all compounds." No — it says for one specific compound, the ratio is fixed. Carbon and oxygen can form both CO and CO₂, each with its own fixed ratio. The law applies separately to each compound.
Quick Check
Sodium chloride (table salt) has the formula NaCl. Atomic masses: Na = 23 u, Cl = 35.5 u. What is the fixed mass ratio of sodium to chlorine in pure salt?
The ratio is 23:35.5, which simplifies to 46:71 (multiplying by 2 to avoid decimals). Every sample of pure NaCl, from any source on Earth, obeys this ratio exactly.
The law of definite (constant) proportions is taught alongside the law of multiple proportions in the same NCERT Class 11 Chemistry foundational chapter, commonly searched as "law of definite proportions definition and example class 11 chemistry".
Water, H2O, is 16/18 oxygen by mass, which works out to 88.9%.
a. 88.9
Step 1. Molecular mass of H2O = 2(1) + 16 = 18 u.
Step 2. Fraction of mass that is oxygen = 16/18 = 0.889, i.e. 88.9%.
Step 3. This matches the law of definite proportions: pure water, from ANY source, always has exactly this same 88.9% oxygen / 11.1% hydrogen composition by mass.
a. 88.9
Divide oxygen's contribution to water's molecular mass (16u) by water's total molecular mass (18u) to get the mass percentage.
- Picking 'b. 18', which is water's molecular mass, not a percentage.
- Computing hydrogen's percentage (11.1%) instead of oxygen's, and picking the wrong option by mixing the two up.
- CBSE 2026Set ANNUAL1 markMCQQ.Dimethyl ether and Ethanol show ________(a) a) Functional isomerism(b) b) Metamerism(c) c) Positional isomerism(d) d) Tautomerism
›Reveal solutionSolution
[!TLDR]
a) Functional isomerism
Why
Both have molecular formula C2H6O but belong to different functional classes (ether vs alcohol) - the definition of functional isomerism.
[!ANSWER]
a) Functional isomerism
- CBSE 2022Set ANNUAL1 markQ.State the law of definite proportion.
›Reveal solutionSolution
The law of definite proportions (Proust's law) states that a given compound always contains exactly the same proportion of elements by mass, no matter its source.
Proposed by the French chemist Joseph Proust in 1799, the law of definite proportions (also called the law of constant composition) states that a given compound always contains exactly the same proportion of elements by mass. For example, pure water, whether obtained from a river, a lake, or prepared synthetically by burning hydrogen in oxygen, always contains hydrogen and oxygen combined in the same fixed mass ratio of 1:8 (a mole ratio of 2:1). This law was fundamental in establishing that compounds have a fixed, reproducible chemical composition, and it later supported Dalton's atomic theory.
✓Final answerLaw of Definite Proportions: a pure chemical compound always contains the same elements combined together in the same fixed proportion by mass, irrespective of its source or method of preparation.
- CBSE 2020Set ANNUAL1 markQ.At constant volume "Pressure of a fixed amount of a gas varies directly with the temperature". Which law is this?
›Reveal solutionSolution
[!TLDR]
Gay-Lussac's Law (Pressure-Temperature Law)
Method
Gay-Lussac's law states that at constant volume, the pressure of a fixed amount of gas is directly proportional to its absolute temperature (P ∝ T).
[!ANSWER]
Gay-Lussac's Law (Pressure-Temperature Law)
- CBSE 2019Set ANNUAL1 markQ.State the law of definite proportion.
›Reveal solutionSolution
The law of definite proportions states that a pure compound always has the same elements in the same fixed mass ratio, regardless of source.
Proposed by the French chemist Joseph Proust in 1799, the law of definite proportions (also called the law of constant composition) states: 'A given compound always contains exactly the same proportion of elements by weight (mass).' For example, pure water (H2O), whether obtained from a river, from rain, or prepared synthetically by burning hydrogen in oxygen, always contains hydrogen and oxygen combined in the fixed mass ratio of 1:8 (i.e. 11.1% hydrogen and 88.9% oxygen by mass). This law was central to establishing Dalton's atomic theory, since a fixed combining ratio by mass is naturally explained if atoms of each element combine in fixed, small whole-number ratios.
✓Final answerLaw of Definite Proportions (Proust's Law): a given chemical compound always contains the same elements combined together in a fixed (definite) proportion by mass, no matter what its source or method of preparation is.
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