1. Percent Composition

2. What is it? 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.

3. How does one calculate it? %mass = grams of element in 1 mol compound x 100% molar mass of compound molar mass of compound Example: What is the percent composition of water (H 2 O)? 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 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 H 2 O x 100 % = 11.21% Hydrogen 16.00 g of O / 18.02 g of H 2 O x 100 % = 88.79% Oxygen Your percentages should add up to 100%

4. 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: Find the mass of the compound 8.20g Mg + 5.40g O = 13.60 g 8.20g Mg + 5.40g O = 13.60 g Find the percent by mass of each element %Mg = 8.20g x 100% = 60.29% of Mg 13.60 g 13.60 g %O = 5.40g x 100% = 39.71 % of O 13.60 g 13.60 g

5. You try it What is the percent composition of glucose (C 6 H 12 O 6 ) figure out the molar mass: 6 mol Carbon x 12.01g = 72.06 g of C 6 mol Carbon x 12.01g = 72.06 g of C 12 mol Hydrogen x 1.01g = 12.12 g of H 12 mol Hydrogen x 1.01g = 12.12 g of H 6 mol Oxygen x 16.00g = 96.00 g of O 6 mol Oxygen x 16.00g = 96.00 g of O molar mass = 180.18 g of glucose molar mass = 180.18 g of glucose Divide the answer of each atom by the molar mass and multiply by 100 72.06 g of C / 180.18 g of glucose x 100% = 39.99 % C 72.06 g of C / 180.18 g of glucose x 100% = 39.99 % C 12.12 g of H / 180.18 g of glucose x 100% = 6.73 % H 96.00 g of O / 180.18 g of glucose x 100% = 53.28 % O

6. Practice Problems Calculate the percentage composition by mass of the following compounds 1) HgO 1) HgO 2) Na 2 S 3) (NH 4 ) 2 S 3) (NH 4 ) 2 S 4) Sr(NO 3 ) 2 1) 92.6% Hg, 7.37% O 1) 92.6% Hg, 7.37% O 2) 58.9% Na, 41.1% S 2) 58.9% Na, 41.1% S 3) 41.1% N, 11.8% H, 47.1% S 3) 41.1% N, 11.8% H, 47.1% S 4) 41.3% Sr, 13.2% N, 45.3% O