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 molecular mass (molecular weight, formula mass, formula weight), MM, of the compound
- Calculate the total mass of each element present in the formula of the compound
- Calculate the percent compositon (percentage composition): % by weight (mass) of element
    = (total mass of element present ÷ molecular mass) x 100
Examples
Calculate the percent by weight of sodium (Na) and chlorine (Cl) in sodium chloride (NaCl)
- Calculate the molecular mass (MM):
MM = 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 weight of Na in NaCl:
%Na = (mass Na ÷ MM) 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 weight of Cl in NaCl:
%Cl = (mass Cl ÷ MM) 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.
Calculate the percent by weight of each element present in sodium sulfate (Na2SO4).
- Calculate the molecular mass (MM):
MM = (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 weight of Na in Na2SO4:
%Na = (mass Na ÷ MM) 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 weight of S present:
%S = (mass S ÷ MM) 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 weight of O in Na2SO4:
%O = (mass O ÷ MM) 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
Calculate the percent by weight of each element present in ammonium phosphate [(NH4)3PO4]
- Calculate the molecular mass (MM) of (NH4)3PO4:
MM = 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 ÷ MM) 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 ÷ MM) 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 ÷ MM) 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 ÷ MM) 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
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