NOTES ON PERCENT COMPOSITION
WHAT IS PERCENT COMPOSITION?
The percent composition by mass of a compound represents the percent that each element in a
compound contributes to the total mass of the compound.
FORMULA:
EXAMPLE 1:
What is the percent composition of water (H2O)?
Step 1: Figure out the molar mass from the formula
2 mol Hydrogen x 1.01g = 2.02g
1mol Oxygen x 16.00g = 16.00g
2.02 g + 16.00 g = 18.02 g/mol H2O
Step 2: Divide the answer for each atom by the molar mass and multiply by 100 to get a
percentage
2.02 g of H
X 100% = 11.21 % H
18.02 g of H2O
16.00 g of O X 100% = 88.79 % O
18.02 g of H2O
Your percentages should
add up to 100%
EXAMPLE 2:
An 8.20g piece of magnesium combines completely with 5.40g of oxygen to form magnesium
oxide. What is the percent composition of the magnesium oxide?
Solution:
Step 1: Find the mass of the compound
8.20g Mg + 5.40g O = 13.60 g
Step 2: Find the percent by mass of each element
8.20g
13.60 g
X 100% = 60.29 % Mg
5.40g
13.60 g
X 100% = 39.71 % O
Your percentages should
add up to 100%
NOTES ON PERCENT COMPOSITION
WHAT IS PERCENT COMPOSITION?
The percent composition by mass of a compound represents the percent that each element in a
compound contributes to the total mass of the compound.
FORMULA:
EXAMPLE 1:
What is the percent composition of water (H2O)?
Step 1: Figure out the molar mass from the formula
2 mol Hydrogen x 1.01g = 2.02g
1mol Oxygen x 16.00g = 16.00g
2.02 g + 16.00 g = 18.02 g/mol H2O
Step 2: Divide the answer for each atom by the molar mass and multiply by 100 to get a
percentage
2.02 g of H
X 100% = 11.21 % H
18.02 g of H2O
16.00 g of O
18.02 g of H2O
Your percentages should
add up to 100%
X 100% = 88.79 % O
EXAMPLE 2:
An 8.20g piece of magnesium combines completely with 5.40g of oxygen to form magnesium
oxide. What is the percent composition of the magnesium oxide?
Solution:
Step 1: Find the mass of the compound
8.20g Mg + 5.40g O = 13.60 g
Step 2: Find the percent by mass of each element
8.20g
13.60 g
X 100% = 60.29 % Mg
5.40g
13.60 g
X 100% = 39.71 % O
Your percentages should
add up to 100%