1. Percent Composition

2. – 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. Steps for Finding Percent Composition 1.Find molecular mass of whole molecule 2.Divide the mass of each type of atom by the molecular mass and multiply by 100 to make into a percent.

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 %Composition= x 100 = % Mass of element Mass of compound

5. Step 2: Find the % of each element by dividing the mass of each element by the total mass Calculate the percentage composition of C 3 H 7 NO 2

6. What is the percent composition of nitrogen in N 2 O?  Step 1: Find the molar mass of N 2 O M r = 2(14.01) + (16.00) = 44.02 g/mol  Step 2: Find the % of each element by dividing the mass of each element by the total mass

7. Calculate the mass of aluminum in 28 g of Aluminum Oxide  Step 1: Calculate the molar mass of aluminum oxide. Al 2 O 3 Mr = 2(26.98) + 3(16.00) = 101.96 g/mol  Step 2: Calculate the % mass of aluminum in aluminum oxide.

8. Calculate the mass of aluminum in 28 g of Aluminum Oxide  Step 3: Find the mass of aluminum in 28g of aluminum oxide using the percent aluminum.

9. % Composition Check Point 1.What is the percent composition of carbon in carbon monoxide? 2.What is the percent composition of oxygen in carbon monoxide? 3.Calculate the mass of oxygen in 42.8g of carbon monoxide.

10. % Composition Check Point 1.What is the percent composition of sulfuric acid (H 2 SO 4 )? 2.What percentage of rubidium in rubidium selenide? 3.Calculate the mass of fluorine in 24.1g of strontium fluoride.