Percent Composition Percent by mass of an element in a compound

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

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 = g  S: 1 x = g  Sum: g = g

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

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 g/mole. Find mass of water:  H: 2 x = g  O: 1 x =  Sum: g/ mole

Sample Problem 7-11 Continued Find percent water in the hydrate: Water: 10 x = g Percent water: g / g = Solution: %

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