# What is the mathematical formula for calculating mass percent composition from a chemical formula?

Oct 7, 2016

Find the molar mass, Find the mass of each element Divide the mass of the element by the molar mass x 100 = %

#### Explanation:

The percent composition is the sum of all the percentages of each element in the compound.

The molecular formula mass is 100% or the total of the compound.
So the first thing is to find the molar mass from the chemical formula.

Then find the mass of each of each element in the compound. For example ethanol has the formula ${C}_{2} {H}_{5} O H$ To find the mass of each element multiply the atomic mass by the number of atoms.

6 x 1 = 6 grams of Hydrogen
2 x 12 = 24 grams of Carbon.
1 x 16 = 16 grams of Oxygen
Total = 45 grams total = the formula mass of the compound.

The divide the mass of each element by the molecular formula mass

$\frac{6}{46}$ = 13% Hydrogen

$\frac{24}{46}$ = 52% Carbon

$\frac{16}{46}$ = 35% Oxygen

To find the percent composition find the molecular mass from the chemical formula of the compound.

Then divide the mass of each element in the compound by the molecular mass and multiply by 100