Unit #7CALCULATIONS BASED ON FORMULAE
7.7 - Percent Composition of Compounds

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
Step 2: Determine the percentages of each element present.

Step 1:

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 = 149.0868 g/mole

Step 2:

Percent composition
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

Note that the sum of the percentages is 100%

 


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