Percent Composition (Percentage Composition)

Key Concepts

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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