

Percent Composition by Mass


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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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