1. Percent Composition
by mass

2. Definition
The percent by mass of each element in a compound

3. Percent by Mass Mass (g) element × 100 Mass (g) compound
**Round each percent to the hundredths place

4. Steps Find the molar mass of your compound
Find the percent composition of each element by using the equation
To check your work, the percent's should add up to 100% exactly, if not go back and check your work

5. Molar Mass of Compound Al = 26.98g
Example
Determine the percent composition of each element of aluminum chloride, AlCl3
%Al = g
133.33g
× 100 = % Al
Molar Mass of Compound Al = 26.98g
+ Cl = 35.45g × 3
133.33g
%Cl = g x3
133.33g
× 100 = % Cl

6. Example
Determine the percent composition of each element of H2SO4? (What is its name?)

7. Example
Determine the percent composition of each element of Ca(OH)2 (What is its name?)