1. % Formula Mass and % Composition
Chemistry

2. % Composition from Mass
% Mass = g of element X 100 g of compound Example: When a g sample of a compound containing magnesium and oxygen is decomposed, 5.40 g of oxygen is obtained. What is the % composition of this compound?

3. Knowns: mass of compound = 13. 60g mass of oxygen = 5
Knowns: mass of compound = 13.60g mass of oxygen = 5.40 g mass of magnesium = – 5.40 = 8.20 g %Mg = 8.20g X 100 = 60.3% 13.60g %O = 5.40g X 100 = 39.7%

4. % Composition from Formula
% mass = g of element in 1 mol cmpd X 100 mass of compound Calculate the % composition of C3H8

5. Knowns: mass of C in 1 mol C3H8 = 36. 0g mass of H in 1 mol C3H8 = 8
Knowns: mass of C in 1 mol C3H8 = 36.0g mass of H in 1 mol C3H8 = 8.0g molar mass of C3H8 = 44.0g/mol % C = 36.0g X 100 = 81.8% 44.0g % H = 8.0g X 100 = 18.2%

6. % Composition as Conversion Factor
You can use % composition to calculate the number of grams of any element in a specific mass of a compound. How much carbon and hydrogen are in 82.0 g of C3H8?

7. Knowns: % C = 81. 8% % H = 18. 2% mass C3H8 = 82. 0g 82. 0 g C3H8 X 81
Knowns: % C = 81.8% % H = 18.2% mass C3H8 = 82.0g 82.0 g C3H8 X 81.8 g C = 67.1 g C 100 g C3H g C3H8 X 18.2 g H = 14.9 g H