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Percent Composition by Mass

 

on to Empirical Formula Calculations

 

Total mass of the Element  (Part)

  x 100% = Percent Composition

Total mass of compound (Whole)

 
Example 1

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.

 

Example 2

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

on to Empirical Formula Calculations

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