Percent Composition from Formulas
The formula of a compound tells us the numbers of atoms of each element in a unit of the compound, or if we use the molar interpretation, the number of moles of each element in one mole of the substance. With this information we can determine the ratio of the mass of each element to the mass of the compound. Multiply these ratios by 100% and you have the mass percent composition of the compound. As an example we will determine the mass percent composition of Al(BrO3)3.
First, determine the molar mass of Al(BrO3)3.
| Formula wt. = at. wt. Al + 3 x at. wt. Br + 9 x at. wt. O |
| = 26.98 u + (3 x 79.90)u + (9 x 16.00)u |
| = 26.98 u + 239.70 u + 144.00 u = 410.68 u |
| Molar mass = 410.68 g/mol Al(BrO3)3 |
Then, for one mole of compound, determine mass ratios and percents.
| % Al = (26.98 g Al / 410.68 g Al(BrO3)3) x 100% = 6.57% |
| % Br = (239.70 g Br / 410.68 g Al(BrO3)3) x 100% = 58.37% |
| % O = (144.00 g O / 410.68 g Al(BrO3)3) x 100% = 35.06% |