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Percent Composition

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

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Percent Composition

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

  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: Cu2S • 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 (Na2CO3 . 10H2O), 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%

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