1. – the percent by mass of each element in a compound.
Percent composition
– the percent by mass of each element
in a compound.
% mass = grams of element x 100%
grams of compound
% mass = g of element in 1 mol of compound x100%
molar mass (in g) of compound

2. Percent Composition from Formula
If the formula is known, a two-step process is
needed to determine the percent composition.
Step 1: Calculate the molar mass
(sum of atomic masses)
Step 2: Divide the total mass of each element
in the formula by the molar mass and
multiply by 100.

3. Example:
Calculate the percent composition of sodium chloride.
Step 1: First calculate the molar mass of NaCl
23.0 g/mol Na
g/mol Cl
58.5 g/mol NaCl
Step 2: Now determine the percent composition.
23.0 g/mol Na x 100% = % Na
58.5 g/mol NaCl
35.5 g/mol Cl x 100% = % Cl
58.5 g/mol NaCl

4. Practice: Calculate the percent composition of Ca(NO3)2. Ca 40.1
40.1 / = 24.4 %
14.0 x 2    N   28.0       
28.0 / 164.1 = 17.1 %
16.0 x 6    O   96.0       
96.0 / 164.1 = 58.5 %
Total 

5. Practice: Calculate the percent composition of K2CrO4. % Comp.
39.1 x 2    K   78.2    
78.2 / = 40.3 %
52.0 / 194.2 = 26.8 %
Cr  52.0    
16.0 x 4    O   64.0    
64.0 / = 33.0 %
Total 

6. Percent Composition from Experimental Data
The percent composition can be determined from
experimental data without knowing the formula of a
compound.
Step 1: Calculate the mass of the compound formed
Step 2: Divide the mass of each element by the total
mass of the compound and multiply by 100.

7. Example: When heated in air, 1.63 g of Zn combines
with 0.40 g of O2 to form zinc oxide. Calculate the
percent composition of the compound formed.
Step 1: Calculate the mass of the compound formed
1.63 g Zn
g O2
2.03 g ZnO
Step 2: Divide the mass of each element by the total
mass of the compound and multiply by 100.
1.63 g Zn x 100% = % Zn
2.03 g ZnO
0.40 g O2 x 100% = % O
2.03 g ZnO

8. Practice: 13.43 g Al 53.18 g Cl 66.61 g Total
Aluminum chloride is formed by reacting g aluminum
with g chlorine. What is the percent composition?
13.43 g   Al
53.18 g   Cl
 66.61 g Total 
13.43 g   Al   X   100   =   % Al
             g
53.18 g   Cl   X   100   =   % Cl
             66.61 g