PERCENT COMPOSITION
Percent Composition
• Percent Composition – the
percentage by mass of
each element in a
compound
Part
_______
Percent =
x 100%
Whole
Percent composition Mass of element in 1 mol
of a compound or = ____________________ x 100%
Mass of compound 1 mol
molecule
CALCULATING PERCENTAGE COMPOSITION

Step 1 : Calculate molar mass

Step 2 : Divide the subtotal for each element’s
mass by the molar mass.

Step 3: Multiply by 100 to convert to a
percentage.
Percent Composition
Example: What is the percent composition of
Potassium Permanganate (KMnO4)?
Molar Mass of KMnO4
K = 1(39.1) = 39.1
Mn = 1(54.9) = 54.9
O = 4(16.0) = 64.0
MM = 158 g
Percent Composition
Example: What is the percent composition of
Potassium Permanganate (KMnO4)?
Molar Mass of KMnO4
% K
39.1 g K
158 g
= 158 g
x 100 = 24.7 %
54.9 g Mn x 100 = 34.8 %
% Mn 158 g
K=
1(39.10) = 39.1
Mn = 1(54.94) = 54.9
O = 4(16.00) = 64.0
MM = 158
64.0 g O x 100 = 40.5 %
% O
158 g
Percent Composition
Determine the percentage composition of sodium carbonate
(Na2CO3)?
Molar Mass
Na = 2(23.00) = 46.0
C = 1(12.01) = 12.0
O = 3(16.00) = 48.0
MM= 106 g
Percent Composition
46.0 g
% Na =106 g
12.0 g
% C = 106 g
48.0 g
% O = 106 g
x 100% = 43.4 %
x 100% = 11.3 %
x 100% = 45.3 %
PERCENT COMPOSITION
What is the % composition of C6H12O6?
Carbon
6(12.011 ) = 72.066 g C
72.066 x100%=40.0% C
180.155
12.0948 x100%=6.7% H
Hydrogen 12(1.0079g =
) 12.0948 g H
180.155
Oxygen
6(15.999g ) = 95.994g O
95.994 x100%=53.3% O
180.155
180.155
% composition is 40.0% C, 6.7% H, and 53.3% O
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EXERCISE
Morphine, derived from opium plants, has
the potential for use and abuse. It’s
formula is C17H19NO3. What percent, by
mass, is the carbon in this compound?
a)
b)
c)
d)
12.0 %
54.8 %
67.9 %
71.6 %
17  12.01
17  12.01 + 19  1.008  + 14.01 +  3  16.00 
= 0.716  100 = 71.6% C
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EXERCISE
Consider separate 100.0 gram samples of each
of the following:
H2O, N2O, C3H6O2, CO2
18g/mol

44g/mol
74g/mol
44g/mol
Rank them from highest to lowest percent
oxygen by mass.
H2O, CO2, C3H6O2, N2O
16/18 32/44 32/74 16/44
0.89 0.73 0.43 0.36
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PERCENT COMPOSITION
The percent composition can also be calculated from
experimental data.
A student prepares a compound of tungsten chloride from
3.946 g of tungsten and 3.806 g of chlorine. Assuming
the reaction goes to completion, calculate the percent
composition.
%W = (mass W / mass WxCly )100 = (3.946 g / 7.752 g)100 =
50.9%
%Cl = (Cl / WxCly )100 = (3.806g / 7.752 g)100 = 49.1%
GROUP STUDY PROBLEMS
______1. A 5.016 g sample of chromium, Cr, combined
with oxygen to give 7.333 g of an oxide compound. The
percent composition of this compound is:
a) 59.1% Cr, 40.9% O
b) 24.4% Cr, 75.6% O
c) 68.4% Cr, 31.6% O
d) 71.5% Cr, 28.5% O
2. Calculate the percent composition.
K2CrO4
K =_____
Cr =_____
O = _____
K2Cr2O7
O = _____
K =_____
Cr =_____