Percentage Composition

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

1. Calculate the molecular mass MM, of the compound
2. Calculate the total molar mass of each element present in the formula of the compound
3. Calculate the percent composition (percentage composition)

             total mass of element present ÷ molecular mass) x 100
Example:
NaCl

• Calculate the molecular mass (MM):
  MM = 22.99(Na) + 35.45(Cl) = 58.44g/mol
• Calculate the total molar mass of Na present:
  1 Na is present in the formula, molar mass = 22.99g/mol
• Calculate the percent by weight of Na in NaCl:
  %Na = (molar 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, molar mass = 35.45g/mol
• Calculate the percent by weight of Cl in NaCl:
  %Cl = (molar 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 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


  How to calculate percent composition?
Hmm...
(: