1. Percentage Composition

2. Percent Composition Compounds are ALWAYS composed of elements in fixed ratios. This is often referred to as the Law of Definite Proportions. This law states that elements are always present in the same proportions by mass in a compound. I.e. H 2 O is always 2H:1O any other proportions would not form water. H 2 O 2 is hydrogen peroxide Since compounds always form in fixed proportions, we can calculate the Percent Composition of an element in a compound. %Composition= x 100 = % Mass of element Mass of compound

3. Example 1: A sample of a compound is analyzed and found to contain 0.90g of Ca and 1.60g of Cl. The sample has a mass of 2.50g. Find the % composition of the compound.

4. Calculate the percentage composition of C 3 H 7 NO 2 Step 1: Find the molar mass of C 3 H 7 NO 2 M = 3(12.01) + 7(1.01) + (14.01) + 2(16.00) = 89.11 g/mol

5. Step 2: Find the % of each element by dividing the mass of each element by the total mass %C = x 100 = 40.43 % %H = x 100 = 7.93% %N = x 100 = 15.72% %O = x 100 = 35.91 % 3(12.01) 89.11 7(1.01) 89.11 1(14.01) 89.11 2(16.00) 89.11

6. What is the percent composition of nitrogen in N 2 O? Step 1: Find the molar mass of N 2 O M = 2(14.01) + (16.00) = 44.02 g/mol

7. Step 2: Find the % of each element by dividing the mass of each element by the total mass %N = x 100 = 63.65% %O = x 100 = 36.35 % 2(14.01) 44.02 1(16.00) 44.02

8. Calculate the mass of aluminium in 28 g of Aluminium Oxide Step 1: Calculate the molar mass of aluminium oxide. Al 2 O 3 M = 2(26.98) + 3(16.00) = 101.96 g/mol

9. Step 2: Calculate the % mass of aluminium in aluminium oxide. %Al = x 100 = 52.92% 2(26.98) 101.96

10. Step 3: Find the mass of aluminium in 28g of aluminium oxide. m Al = x 28g = 15g 52.92 100

11. Alternate Solution =Al on top Total on bottom = m Al = 15 g M Al M Total m Al m Total 2(26.98) 101.96 m Al 28g