1. Percent Composition Percent by mass of an element in a compound

2. Percent Composition Divide mass of the element by formula mass Multiply the decimal times 100 to get percent

3. Sample Problem 7-10 Find percent composition of copper (I) sulfide First write formula:  Cu +1 bonds with S -2  Formula: Cu 2 S Find formula mass:  Cu: 2 x 63.55 = 127.1 g  S: 1 x 32.07 = 32.07 g  Sum: 159.17 g = 159.2 g

4. Sample Problem 7-10 continued Find percentages:  Copper: 127.1 g/159.2 g = 0.7983668 = 79.84%  Sulfur: 32.07 g/159.2 g = 0.20144472 = 20.14% Check!  Add 79.84 + 20.14 = 99.98% (close to 100%)

5. Sample Problem 7-11 Sodium carbonate forms a hydrate, which bonds 10 water molecules to the ionic compound. Find the mass percent of water in sodium carbonate decahydrate (Na 2 CO 3. 10H 2 O), which has a molar mass of 286.14 g/mole. Find mass of water:  H: 2 x 1.0079 = 2.0158 g  O: 1 x 15.9994 = 15.9994  Sum: 18.015 g/ mole

6. Sample Problem 7-11 Continued Find percent water in the hydrate: Water: 10 x 18.015 = 180.15 g Percent water: 180.15 g / 286.14 g = 0.6295939 Solution: 62.959%

7. Now you try it! Calculate the percent by mass of sodium and chlorine in sodium chloride Sodium: 39.34% Chlorine: 60.66%