One 100 g sample comprised of molecule (A) is made up of 57.1 g of oxygen and 42.9 g of carbon. Another 100 g sample comprised of molecule (B) is made up of 72.7 g oxygen and 27.3 g carbon. How does the oxygen to carbon mass ratio for molecule A compare to the oxygen to carbon mass ratio for molecule B?

The first compound contains 42.9% by mass carbon and 57.1% by mass oxygen. The second compound contains 27.3% by mass carbon and 72.7% by mass oxygen.

The law of multiple proportions states that the masses of one element which combine with a fixed mass of the second element are in a ratio of whole numbers.

Therefore, the masses of oxygen in the two compounds that combine with a fixed mass of carbon should be in a whole number ratio. In 100 grams of the first compound there are 57.1 grams oxygen and 42.9 grams carbon. The mass of oxygen (O) per gram of carbon (C) is:

57.1 g O / 42.9 g C = 1.33 g O per g C

In molecule B, there are 72.7 grams of oxygen (O) and 27.3 grams of carbon (C). The mass of oxygen per gram of carbon is:

72.7 g O / 27.3 g C = 2.66 g O per g C

Comparing the mass of oxygen (O) per gram of carbon (C) of both molecules, we have;

2.66 / 1.33 = 2

The Combination ratio is 2: `1