Percentage composition by mass is the percentage of the total mass of a compound contributed by each element, found by dividing the mass of that element in the formula by the molar mass and multiplying by one hundred.
Percentage composition by mass is the percentage of the total mass of a compound contributed by each element. You find it by dividing the mass of that element in the formula by the molar mass of the compound and multiplying by one hundred. Malay: komposisi peratus mengikut jisim · Chinese: 质量百分组成.
This quantity tells you how much of a compound’s mass comes from each element, which is useful for comparing fertilisers by their nitrogen content or judging how much metal an ore contains. To calculate it you first work out the relative formula mass, then treat each element’s contribution as a fraction of that total.
Example. In water, H2O, the two hydrogen atoms contribute a mass of 2 and the oxygen contributes 16, out of a total of 18. So the percentage of hydrogen is (2 ÷ 18) × 100, which is about 11 percent, and the percentage of oxygen is (16 ÷ 18) × 100, which is about 89 percent. The percentages of all the elements in a compound should add up to one hundred, and this makes a useful check on your working.
Confusion to avoid. Remember to multiply each relative atomic mass by the number of atoms shown in the formula before dividing, because forgetting a subscript is a common slip. Do not confuse percentage composition with the empirical formula: they are related, since percentages can be turned into a mole ratio, but percentage composition is about mass while the empirical formula is about the ratio of atoms.
In this chapter percentage composition by mass is one of the standard calculations built on the relative formula mass, and it is a stepping stone towards finding an empirical formula from experimental data across SPM Chemistry.
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