medium difficulty7 min

Law of Constant Proportions

The law of constant proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass.

In one line: Pure compound, fixed mass ratio.

Practice This Concept

Understand it

A compound has a definite composition. Water from any pure source contains hydrogen and oxygen in the same mass ratio, 1:8. The source or method of preparation does not change this fixed proportion.

Example

In pure water, 2 g of hydrogen combines with 16 g of oxygen. The ratio by mass is 2:16, or 1:8.

Rule to remember

For a pure compound, mass of element A : mass of element B is constant for every pure sample.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Sample 1 has 3 g carbon and 8 g oxygen.

1Sample 2 has 6 g carbon and 16 g oxygen.
2Both give the ratio 3:8, so they can represent the same pure compound.

Method you can reuse

1Write the masses of elements.
2Reduce each sample to simplest ratio.
3Compare the ratios.
4If they match, the law is followed for the same pure compound.

Visual tip

A two-column table comparing two pure samples can show that their simplified mass ratios are identical.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does pure water always have hydrogen and oxygen in the mass ratio 1:8?

Think first, then show answer

Pure water always has the formula H₂O, so its elements combine in a fixed composition. When atomic masses are considered, hydrogen and oxygen are present in the mass ratio 1:8.

Common wrong answer

Saying 6:16 is different from 3:8.

Why it loses marks: 6:16 simplifies to 3:8.

Don't mix this up

Students sometimes compare the number of atoms directly with the mass ratio. Atomic mass must be considered while finding mass proportion.

Exam tip

For mass-ratio questions, reduce the given masses to the simplest ratio and check whether the ratio is constant.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked through sample comparison, fixed ratio calculation, or a reason why a compound has definite composition.

1
Enough for 1 mark
The law of constant proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass.
2
Add for 2 marks
Example: In pure water, 2 g of hydrogen combines with 16 g of oxygen. The ratio by mass is 2:16, or 1:8.
3
Add for 3 marks
A compound has a definite composition.

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