PERCENT COMPOSITION
• The percent composition is the mass percentage of each type of
atom(element) in a compound.
% X = (total atomic mass of X / molar mass which contains X)
For Example:
Calculate the percent composition of nicotine,
C10H14N2.
 Molar mass = 10C + 14H + 2N = 162 g/mol
 %C = (10C / C10H14N2)100 = (120 g/mol /162
g/mol)100=74.1%
 %H = (14H / C10H14N2)100 = (14 g/mol / 162 g/mol)100 =
8.6%
 %N = (2 N / C10H14N2)100 = (28 g/mol / 162 g/mol)100 =
17.3%
PERCENT COMPOSITION
• The percent composition can also be calculated from
experimental data.
% X = (total mass of X / total mass of compound)
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.
 According to the Law of Conservation of Mass, 3.946 g
of W combines with 3.806 g of Cl to give 7.752 g of
compound.
 %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%
PERCENT COMPOSITION
WORD PROBLEMS
Q. How many grams of lithium will combine with 20.0 g of
sulfur to form Li2S?
 Molar mass of Li2S = 2Li + S = 2(6.94 g/mol) + 32.06
g/mol =45.94 g/mol
 % composition of Li in Li2S = (mass of Li/molar
mass)100 =
(13.88 g/mol / 45.94 g/mol) 100 = 30.2%
 % composition of S in Li2S = 100% - 30.2% =69.8%
 A ratio can now be established:
20.0 g S = 69.8%
x = (30.2%) (20.0 g)
x = 8.65 g
x g Li
30.2%
69.8%
PRACTICE PROBLEMS #11
A
_____1.
A 45.9 g sample of Copper, Cu, combined with
oxygen to give 51.69 g of an oxide compound. The
percent composition of this compound is:
a) 88.8% Ca, 11.2% O
b) 24.4% Ca, 75.6% O
c) 60.1% Ca, 39.9% O
d) 11.3% Ca, 88.7% O
2. Calculate the percent composition for each set?
11.1%
88.9%
H2O
H =_____
O = _____
H2O2
5.88%
H =_____
94.1%
O = _____
3. How many grams of sodium will combine with 567.0 g
of sulfur to form Na2S?.
815.6 g
GROUP STUDY PROBLEMS #11
______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 =_____
3. How many grams of aluminum will combine with 3.67 g
of sulfur to form Al2S3?