Percent Composition Notes
Percent composition is the percent by
mass of each element present in a
compound.
Water, H2O, is the first example.
One mole of water is 18.0152 grams.
In that compound, there are two moles
of H atoms and 2 x 1.008 = 2.016
grams. That's how many grams of
hydrogen are present in one mole of
water.
There is also one mole of oxygen atoms
weighing 16.00 grams in the mole of
water.
To get the percentage of hydrogen,
divide the 2.016 by 18.015 and
multiply by 100, giving 11.19%.
For oxygen it is (16.00 ÷ 18.015) x 100
= 88.81%.
Notice that you can also minus
hydrogen's percentage from 100%.
You can always do this with the last
part of this type problem.
Glucose, C6H12O6, is the second
example.
1. Figure out the molar mass from the
formula.
2. Figure out the grams each atom
contributes by multiplying the atomic
weight by the subscript.
3. Divide the answer for each atom by
the molar mass and multiply by 100 to
get a percentage.
Remember, you may figure out the
last percentage by subtracting the
total percent from 100, as will be done
in a moment.
Step One: mass of one mole of glucose
= 180.16 g
Step Two:
Carbon = 6 x 12.011 g = 72.066 g
Hydrogen = 12 x 1.008 = 12.096 g
Oxygen = 6 x 16.00 = 96.00 g
Step Three:
Carbon's percentage: (72.066 g / 180.16
g) x 100 = 40.00 %
Hydrogen's percentage: (12.096 g /
180.16 g) x 100 =6.71 %
Oxygen's percentage: 100 - (40.00 +
6.71) = 53.29 %
You may calculate the oxygen
percentage if you wish.
Please calculate the percent composition
of the following substances before going
to percent composition: part two:
CH2O
CH3COOH
Practice Problems
Calculate the percent composition for
each compound listed.
1) KNO3
2) H2SO4
3) C2H5OH
4) C6H5NH2
Answers
1) KNO3
Molar mass = 101.1 g/mol
Potassium: (39.10 / 101.1) x 100 =
38.67%
Nitrogen: (14.01 / 101.1) x 100 =
13.86%
Oxygen: (48.00 / 101.1) x 100 = 47.48%
2) H2SO4
Molar mass = 98.07 g/mol
Hydrogen: (2.016 / 98.07) x 100 =
2.06%
Sulfur: (32.06 / 98.07) x 100 = 32.69%
Oxygen: (64.00 / 98.07) x 100 = 65.26%
3) C2H5OH
Molar mass = 46.07 g/mol
Carbon: (24.022 / 46.07) x 100 = 52.14%
Hydrogen: (6.048 / 46.07) x 100 =
13.13%
Oxygen: (16.00 / 46.07) x 100 = 34.73%
4) C6H5NH2
Molar mass = 93.13 g/mol
Carbon: (72.066 / 93.13) x 100 = 77.38%
Hydrogen: (7.056 / 93.13) x 100 =
7.58%
Nitrogen: (14.01 / 93.13) x 100 =
15.04%
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