How does the law of definite proportions apply to compounds?

The law of definite proportions states that a compound always contains exactly the same proportion of by mass.

This means that the composition of a compound does not depend on its source or its method of preparation.

In water, for example, the mass of oxygen must always have the same ratio to the mass of hydrogen (mass ratio is ##"O:H = 7.94:1"##).

To tell if containing the same elements are the same or different, we can determine the ratios of their masses.

EXAMPLE

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

Solution

Compound 1: ##"Mass ratio of O:H" = (120.0 color(red)(cancel(color(black)("g"))))/(15.0 color(red)(cancel(color(black)("g")))) = 8.00##

Compound 2: ##"Mass ratio of O:H" = (32.0 color(red)(cancel(color(black)("g"))))/(2.00 color(red)(cancel(color(black)("g")))) = 16.0##

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