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Percent Composition (Percentage Composition)

Key Concepts

The percent composition (percentage composition) of a compound is a relative measure of the mass of each different element present in the compound.

To calculate the percent composition (percentage composition) of a compound:

Calculate the relative molecular mass (molecular weight, formula mass, formula weight), M_r, of the compound
Calculate the total mass of each element present in the formula of the compound
Calculate the percentage compositon : % by weight (mass) of element
= (total mass of element present ÷ molecular mass) x 100

Examples

Example 1

Calculate the percent by mass (weight) of sodium (Na) and chlorine (Cl) in sodium chloride (NaCl)

Calculate the relative molecular mass (M_r):
M_r = 22.99 + 35.45 = 58.44
Calculate the total mass of Na present:
1 Na is present in the formula, mass = 22.99
Calculate the percent by mass (weight) of Na in NaCl:
%Na = (mass Na ÷ M_r) x 100 = (22.99 ÷ 58.44) x 100 = 39.34%
Calculate the total mass of Cl present:
1 Cl is present in the formula, mass = 35.45
Calculate the percent by mass (weight) of Cl in NaCl:
%Cl = (mass Cl ÷ M_r) x 100 = (35.45 ÷ 58.44) x 100 = 60.66%

The answers above are probably correct if %Na + %Cl = 100, that is,
39.34 + 60.66 = 100.

Example 2

Calculate the percent by mass (weight) of each element present in sodium sulfate (Na₂SO₄).

Calculate the relative molecular mass (M_r):
M_r = (2 x 22.99) + 32.06 + (4 x 16.00) = 142.04
Calculate the total mass of Na present:
2 Na are present in the formula, mass = 2 x 22.99 = 45.98
Calculate the percent by mass (weight) of Na in Na₂SO₄:
%Na = (mass Na ÷ M_r) x 100 = (45.98 ÷ 142.04) x 100 = 32.37%
Calculate the total mass of S present in Na₂SO₄:
1 S is present in the formula, mass = 32.06
Calculate the percent by mass (weight) of S present:
%S = (mass S ÷ M_r) x 100 = (32.06 ÷ 142.04) x 100 = 22.57%
Calculate the total mass of O present in Na₂SO₄:
4 O are present in the formula, mass = 4 x 16.00 = 64.00
Calculate the percent by mass (weight) of O in Na₂SO₄:
%O = (mass O ÷ M_r) x 100 = (64.00 ÷ 142.04) x 100 = 45.06%

The answers above are probably correct if %Na + %S + %O = 100, that is,
32.37 + 22.57 + 45.06 = 100

Example 3

Calculate the percent by mass (weight) of each element present in ammonium phosphate [(NH₄)₃PO₄]

Calculate the relative molecular mass (M_r) of (NH₄)₃PO₄:
M_r = 3x[14.01 + (4 x 1.008)] + 30.97 + (4 x 16.00) = 3 x [14.01 + 4.032] + 30.97 + 64.00 = (3 x 18.042) + 30.97 + 64.00 = 54.126 + 30.97 + 64.00 = 149.096
Calculate the total mass of N present:
3 N are present, mass = 3 x 14.01 = 42.03
Calculate the percent by mass of N present in (NH₄)₃PO₄:
%N = (mass N ÷ M_r) x 100 = (42.03 ÷ 149.096) x 100 = 28.19%
Calculate the total mass of H present:
12 H are present in the formula, mass = 12 x 1.008 = 12.096
Calculate the percent by mass of H present in (NH₄)₃PO₄:
%H = (mass H ÷ M_r) x 100 = (12.096 ÷ 149.096) x 100 = 8.11%
Calculate the total mass of P present:
1 P is present in the formula, mass = 30.97
Calculate the percent by mass P in (NH₄)₃PO₄:
%P = (mass P ÷ M_r) x 100 = (30.97 ÷ 149.096) x 100 = 20.77%
Calculate the total mass of O present:
4 O are present in the formula, mass = 4 x 16.00 = 64.00
Calculate the percent by mass of O in (NH₄)₃PO₄:
%O = (mass O ÷ M_r) x 100 = (64.00 ÷ 149.096) x 100 = 42.93%

The answers above are probably correct if %N + %H + %P + %O =100, that is,
28.19 + 8.11 + 20.77 + 42.93 = 100

Practice Questions

Homework Checker

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Click on the Percentage Composition drill link:
Percentage Composition drill
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Click the "New Question" button to begin the drill.
Worked solutions are provided if you need some help!

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For AUS-e-TUTE members:
1. Enter the total number of atoms of each element present.
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3. The percentage of each element present will appear.

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