1. PERCENT COMPOSITION
CHAPTER 7-3

2. Percent Composition
The relative amounts of each element in a compound are expressed as percent composition or by the percent composition by mass of each element in a compound.
To calculate the percent composition of a compound, we must use the following formula:
Molar mass element
% Composition = x 100
Molar mass compound
Percent composition
Percent mass
How to calculate percent composition

3. Sample Problem Possible Test Question (PTQ)
Calculate the percent composition of CH3OH.
Find the molar mass of the compound.
C = 1 x 12 = 12
H = 4 x 1 = 4
O = 1 x 16 = 16
32 g/mol

4. Use % composition formula.
12 g/mol
% C = x 100 = 37.5% C
32 g/mol
4 g/mol
% H = x 100 = 12.5% H
16 g/mol
% O = x 100 = 50% O

5. Percent Composition by Mass
How to calculate the percent mass of an element in a compound?
The percent by mass of an element in a compound is the number of grams of the element divide by the number of grams of the compound, multiplied by 100.
grams of element
% mass = x 100
grams of compound

6. Sample Problem Possible Test Question (PTQ)
Law of Conservation of mass
An 8.20 piece of magnesium combines completely with 5.40 g of oxygen to form a compound. What is the percent composition of this compound?
Find the mass of the compound.
2 Mg O2  MgO
8.20 g g = g

7. Use % mass formula
8.20 g Mg
% Mg = x 100 = % Mg
13.6 g MgO
5.40 g O
% O = x 100 = % O