# Percent Composition

## Key Questions

• Percent composition tells you which types of atoms (elements) are present in a molecule and their levels. Percent composition can also tell you about the different elements present in an ionic compound as well.

Let's look at two examples

The molar mass of ${H}_{2} O$ is 18 g/mol
The hydrogens make up 2g (since each mole of hydrogen is 1g)
The oxygen makes up 16g.

The percent composition of the compound is:
H = (2g/18g) x 100 = 11.1%
O = (16g/18g) x 100 = 88.9%

Percent composition can be calculated the chemical formula of a compound, or it can be determined experimentally.

Here is a video which discusses how to calculate percent composition from experimental data for a reaction of iron and oxygen which produces an iron oxide compound.
Video from: Noel Pauller

Hope this helps!

• Here is an example for the compound C2HO4

You will first need to find the molar mass of the compound.

C = 2 x 12 = 24
O = 1 x 16 = 16
H = 4 x 1 = 4
Total mass = 40 amu

To find the percent composition you divide each part by the whole and multiply by 100 to convert to a %.

C = (24/44) x 100 = 54.5%
O = (16/44) x 100 = 36.4%
H = (4/44) x 100 = 9.1%

Percent composition can also be calculated using experimental data. The video below shows you how to do that type of calculation.

• Percent composition in chemistry typically refers to the percent each element is of the compound's total mass.

The basic equation = mass of element / mass of compound X 100%

For instance, if you had a 80.0 g sample of a compound that was 20.0 g element X and 60.0 g element y then the percent composition of each element would be:

Element X = 20.0 g X / 80.0 g total x 100% = .250 or 25.0 %
Element Y = 60.0 g Y / 80.0 g total x 100 % = .750 or 75.0 %

Here is a video which discusses how to calculate percent composition from experimental data for a reaction of iron and oxygen which produces an iron oxide compound. Video from: Noel Pauller