1. 10.3 Percent Composition
Def: Relative amounts of the different elements in a compound
What percent of water is hydrogen? oxygen?

2. Different compounds have different percentages of the same elements
Based on the chemical formula or recipe

3. Formula to calculate percent mass:
Percent mass = mass of element
of element mass of compound
X 100%

4. Sample problems: from mass
1. When a g sample of a compound with magnesium oxide is decomposed, 5.4 g of oxygen is obtained. What is the percent composition of this compound?
First find total mass
Next find mass of each element
Answer: 60.3 % Mg and 39.7 % O

5. 2. Calculate the percent composition when 13. 3 g of Fe combine with 5
2. Calculate the percent composition when 13.3 g of Fe combine with 5.70 g of O.
Find total mass.
Find mass of each element.
Answer : 70.0 % Fe and 30.0% O

6. Sample problems: from formula
1. Calculate the percent composition of propane, C3H8.
Find the total mass
Find the mass of each element
Answer: 82% C and 18% H