Law of Definite Proportions

The law of definite proportions says a compound always has the same mass ratio of its elements, no matter how it is made. Water (H₂O) fixes hydrogen to oxygen at 1:8 by mass, and copper oxide (CuO) fixes copper to oxygen at 4:1. Any amount past that fixed ratio does not react and is left over, so leftover mass is a common exam question. On this page, change the copper mass to see how much oxide forms and what remains.

Compounds Combine in Fixed Ratios

🧩 Intuition — Same Recipe Every Time

①Water (H₂O) is always 2 hydrogen + 1 oxygen

②No matter who makes it or where, the ratio is the same

③That is the law of definite proportions — discovered by Proust

Cu Mass16 g

What Happens to Excess?

🔍 Unreacted Substance

①16 g Cu + 4 g O → 20 g CuO (perfect)

②24 g Cu + 4 g O → 20 g CuO + 8 g leftover Cu

③Anything beyond the fixed ratio stays unreacted

④Tests often ask for the leftover mass!

Summary

Law of Definite Proportions

Element mass ratio in a compound is always fixed

e.g., in CuO, Cu:O = 4:1 (by mass)

Composition of Water

H₂O → H : O = 1 : 8 (by mass)

1 g H + 8 g O → 9 g water

🎯 Exam Points

①Definite proportions: element mass ratio is always constant

②H₂O: H:O = 1:8

②CuO: Cu:O = 4:1

④Excess reactant remains unreacted

⑤The bend in a mass graph = complete-reaction point

Examples and Unit Test

Example 1

In copper(II) oxide (CuO), copper and oxygen always combine in the mass ratio 4 : 1. To react 16 g of copper completely, how many grams of oxygen are needed, and how many grams of copper oxide form?

Since Cu : O = 4 : 1, the oxygen needed for 16 g of copper = 16 ÷ 4 = 4 g.

The copper oxide formed = 16 g + 4 g = 20 g (mass conserved).

▸ 4 g of oxygen needed / 20 g of copper oxide formed

By the law of definite proportions, the mass ratio of the elements in a compound is always constant.

Example 2

24 g of copper is mixed and reacted with 4 g of oxygen. How many grams of copper oxide form, and what substance is left over and how much? (Cu : O = 4 : 1)

Only 4 × 4 = 16 g of copper reacts with the 4 g of oxygen, so copper oxide = 16 + 4 = 20 g forms.

Of the 24 g of copper added, only 16 g reacts, so copper 24 − 16 = 8 g is left over unreacted.

▸ 20 g of copper oxide forms / 8 g of copper is left over

Any substance beyond the fixed ratio cannot react and is left over.

Unit Test

Which statement about the law of definite proportions is correct?

①The mass ratio of elements in a compound changes with how it is made

②In water (H₂O) the mass ratio of hydrogen to oxygen is 1 : 8

③A substance added beyond the fixed ratio still reacts completely

④The law of definite proportions holds for mixtures too

⑤In copper oxide the mass ratio of copper to oxygen is 1 : 4

▸ ② In water (H₂O) the mass ratio of hydrogen to oxygen is 1 : 8

The mass ratio of elements in a compound is always constant (① wrong), and a substance beyond the fixed ratio cannot react and is left over (③ wrong).

The law holds only for compounds, while a mixture has no fixed ratio (④ wrong), and in copper oxide Cu : O = 4 : 1 (⑤ wrong); in water H : O = 1 : 8 (② correct).

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Mass Conservation

Gas Reaction Law

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