What is Percent Composition? 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.

How do I calculate percent composition? Formula: %mass = grams of element in 1 mol compound x 100% 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. 01 g = 2. 02 g 1 mol Oxygen x 16. 00 g = 16. 00 g molar mass of water = 18. 02 g 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%

Same concept, but worded differently An 8. 20 g piece of magnesium combines completely with 5. 40 g of oxygen to form magnesium oxide. What is the percent composition of the magnesium oxide? Solution: Step 1: Find the mass of the compound 8. 20 g Mg + 5. 40 g O = 13. 60 g Step 2: Find the percent by mass of each element %Mg = 8. 20 g x 100% = 60. 29% of Mg 13. 60 g %O = 5. 40 g x 100% = 39. 71 % of O 13. 60 g

Your Turn: What is the percent composition of glucose (C 6 H 12 O 6) Step 1: figure out the molar mass: Step 2: Divide the answer of each atom by the molar mass and multiply by 100

Practice Problems Calculate the percentage composition by mass of the following compounds 1) 2) 3) 4) Hg. O Na 2 S (NH 4)2 S Sr(NO 3)2