Mass Percent Composition

What is mass percent composition?

In the context of compounds, the percent composition of an element in a compound is the percent, by mass, of an element in a compound.

What are the steps to calculating percent composition?

1. Calculate the molar mass of the compound
2. Calculate the molar mass of the element of interest in the compound by multiplying the subscript by the molar mass of the element
3. Divide the molar mass of the element in the compound by the total molar mass of the compound

What is the percent composition of oxygen in water?

Table 31.3: Calculating percent composition.

Step 1:
Calculate the molar mass of the compound	Molar mass of H$_{2}$O = 18 g/mole	18 g/mole
Step 2:
Calculate the molar mass of the element of interest in the compound by multiplying the subscript by the molar mass of the element	Oxygen has a subscript of 1, or one mole of oxygen per mole of water, therefore, 1 mol oxygen times 16 g/mol oxygen equals 16 g oxygen/mol water	16 g/mole
Step 3:
Divide the molar mass of the element in the compound by the total molar mass of the compound	16 g oxygen/mol water divided by 18 g water/mol water = .89 or 89% composition of one mole of water by oxygen.	16 g oxygen/mol water divided by 18 g water/mol water = .89