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.
How do I calculate percent composition?
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Formula:
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%mass = grams of element in 1 mol compound x 100%
molar mass of compound
Example: 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
molar mass of water = 18.02g
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 / 18.02 g of H2O x 100 % = 11.21% Hydrogen
16.00 g of O / 18.02 g of H2O x 100 % = 88.79% Oxygen
Your percentages should add up to 100%
Same concept, but worded differently

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
%Mg = 8.20g x 100% = 60.29% of Mg
13.60 g
%O = 5.40g x 100% = 39.71 % of O
13.60 g
Your Turn:

What is the percent composition of glucose (C6H12O6)

Step 1: figure out the molar mass:

Step 2: Divide the answer of each atom by the molar mass
and multiply by 100
Practice Problems

Calculate the percentage composition by
mass of the following compounds

1)
2)
3)
4)
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HgO
Na2S
(NH4)2S
Sr(NO3)2