Law of Multiple Proportions

The law states that if two elements A and B combine together to form more than one compound, the various masses of B which combine with a fixed mass of A are in a simple whole-number ratio. e.g. 1:2, 2:1, 1:3, 3:2, etc.

Verification of the Law:

Weigh two clean metal boats, weigh them again, containing pure copper(I) oxide (Cu₂ O) and copper(II) oxide (CuO), respectively. Reduce the oxide to copper by using hydrogen gas as described in the experiment on the Law of Definite Proportion . 

The mass of copper, which combined with a fixed mass of oxygen, is found in both samples. The masses are in a simple ratio.

i. \( \scriptsize Cu_2O_{(s)} \: + \: H_{2(g)} \: \rightarrow \: 2Cu_{(s)} \: + \: H_2O_{(g)} \)

ii. \( \scriptsize CuO_{(s)} \: + \: H_{2(g)} \: \rightarrow \: Cu_{(s)} \: + \: H_2O_{(g)} \)

In order to show that these figures conform to the law of multiple proportions, use 1 g or 100 g as a fixed mass.

In sample I:

0.49 g of oxygen combined with 2.55 g of Cu using 1 g as fixed mass.

\( \frac{2.55g}{0.49g} \: \times \: \frac{1}{1} \\ \scriptsize = 5.20\: g \)

In sample II:

0.53 g of oxygen combined with 1.38 g of copper using 1 g as fixed mass.

\( \frac{1.38g}{0.53g} \: \times \: \frac{1}{1} \\ \scriptsize = 2.6\:g \)

The mass of copper that has combined with a fixed mass