Percent Composition What is it The percent composition

Percent Composition

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.

How does one calculate it? %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: Find the mass of the compound 8. 20 g Mg + 5. 40 g O = 13. 60 g 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

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. 01 g = 72. 06 g of C 12 mol Hydrogen x 1. 01 g = 12. 12 g of H 6 mol Oxygen x 16. 00 g = 96. 00 g of O 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 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

Practice Problems Calculate the percentage composition by mass of the following compounds 1) Hg. O 2) Na 2 S 3) (NH 4)2 S 4) Sr(NO 3)2 1) 92. 6% Hg, 7. 37% O 2) 58. 9% Na, 41. 1% S 3) 41. 1% N, 11. 8% H, 47. 1% S 4) 41. 3% Sr, 13. 2% N, 45. 3% O