
 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 (Na_{2}SO_{4}).  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 Na_{2}SO_{4}:
%Na = (mass Na ÷ MM) x 100 = (45.98 ÷ 142.04) x 100 = 32.37%  Calculate the total mass of S present in Na_{2}SO_{4}:
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 Na_{2}SO_{4}:
4 O are present in the formula, mass = 4 x 16.00 = 64.00  Calculate the percent by weight of O in Na_{2}SO_{4}:
%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 
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