# What is Proust's Law?

Mar 26, 2014

Proust’s Law is now called the Law of Definite Proportions.

#### Explanation:

It states that a compound always contains the same proportion of elements by mass

The Law is saying that water, for example, is always H₂O. There are no “intermediate'' compounds. In water, the mass of oxygen always has the same ratio to the mass of hydrogen (mass ratio is O:H = 7.94:1).

To tell if compounds containing the same elements are the same, we determine the ratios of their elemental masses.

EXAMPLE

Compound 1 contains 15.0 g of hydrogen and 120.0 g oxygen. Compound 2 has 2.00 g of hydrogen and 32.0 g oxygen. Are the compounds the same?

Solution

Compound 1: Mass ratio of O:H = $\left(120.0 \text{ g")/(15.0" g}\right)$ = 8.00

Compound 2: Mass ratio of O:H = $\left(32.0 \text{ g")/(2.00" g}\right)$ = 16.0

The mass ratios are not the same. The compounds are different.