# How is the law of definite composition related to the law of multiple proportions?

Aug 17, 2014

Short answer: The Law of Multiple Proportions says that when two or more elements form different compounds, each of those compounds must obey the Law of Definite Composition.

The Law of Multiple Proportions states that if two elements form more than one compound, the masses of one element that combine with a fixed mass of the second element must be in a small whole-number ratio.

For example, in CO, 12 g of carbon are combined with 16 g of oxygen.

In CO₂, 12 g of carbon are combined with 32 g of oxygen.

The ratios of oxygen are $\left(32 \text{g")/(16"g}\right) = \frac{2}{1}$.

But this implies that CO and CO₂ always have the same composition.

The Law of Definite Composition states that all compounds must have the same composition by mass.

For example, CO always contains 12 g of carbon for every 16 g of oxygen.

And CO₂ always contains 12 g of carbon for every 32 g of oxygen.