1. % X = (total atomic mass of X / molar mass which contains X)
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%

2. % X = (total mass of X / total mass of compound)
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 g of tungsten and g of chlorine. Assuming the reaction goes to completion, calculate the percent composition.
According to the Law of Conservation of Mass, g of W combines with g of Cl to give g of compound.
%W = (mass W / mass WxCly )100 = (3.946 g / g)100 = 50.9%
%Cl = (Cl / WxCly )100 = (3.806g / g)100 = 49.1%

3. 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) g/mol =45.94 g/mol
% composition of Li in Li2S = (mass of Li/molar mass)100 =
(13.88 g/mol / g/mol) 100 = 30.2%
% composition of S in Li2S = 100% % =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 % %

4. PRACTICE PROBLEMS #11 815.6 g A 11.1% 88.9% 5.88% 94.1%
_____1. A 45.9 g sample of Copper, Cu, combined with oxygen to give g of an oxide compound. The percent composition of this compound is:
a) % Ca, % O
b) % Ca, % O
c) % Ca, % O
d) % Ca, % O
2. Calculate the percent composition for each set?
H2O H =_____ O = _____
H2O2 H =_____ O = _____
3. How many grams of sodium will combine with g of sulfur to form Na2S?.
11.1% %
5.88% %
815.6 g

5. GROUP STUDY PROBLEMS #11
______1. A g sample of chromium, Cr, combined with oxygen to give g of an oxide compound. The percent composition of this compound is:
a) % Cr, 40.9% O
b) % Cr, 75.6% O
c) % Cr, 31.6% O
d) % Cr, 28.5% O
2. Calculate the percent composition.
K2CrO4 K =_____ Cr =_____ O = _____
K2Cr2O7 K =_____ Cr =_____ O = _____
3. How many grams of aluminum will combine with 3.67 g of sulfur to form Al2S3?