Percent composition of compounds

The proportion by mass of each element in a compound is called percent composition.

Here is how to determine the percent composition for a compound. Take a look at a sample calculation.

Example

Determine the percent composition for ammonium phosphate, (NH₄)₃PO₄, a compound that is used as a fertilizer.

• Step 1: Determine the molar mass of (NH₄)₃PO₄

3 moles of N atoms	= 3(14.007 g)	= 42.021 g
12 moles of H atoms	= 12(1.008 g)	= 12.096 g
1 mole of P atoms	= 1(30.9738 g)	= 30.9738 g
4 moles of O atoms	= 4(15.999 g)	= 63.996 g

1 mole of (NH₄)₃PO₄ = 149.0868 g

Molar mass of (NH4)3PO4 = 149.0868 g/mole

• Step 2: Determine the percentages of each element present.

N: [42.021 g N / 149.0868 g (NH4)3PO4] x 100%	= 28.19 % N
H: [12.096 g H / 149.0868 g (NH4)3PO4] x 100%	= 8.11 % H
P: [30.9738 g P / 149.0868 g (NH4)3PO4] x 100%	= 20.78 % P
O: [63.996 g O / 149.0868 g (NH4)3PO4] x 100%	= 42.93 % O

To minimize errors due to rounding off numbers, keep as many significant figures in the calculations as possible.

Note that the sum of the percentages is 100%

Content suitability

BCIT courses: CHEM 0011