J Percentage Composition

Sample Problem J Find the percentage composition of copper(I) sulfide, Cu S. 0 ANALYZE 

A good check is to see if the results add up to about 100%. (Because of rounding, the total may not always be exacdy 100%.) 

Sample Problem K As some salts crystallize from a water solution, they bind water molecules in their crystal structure. Sodium carbonate forms such a hydrate, in which 10 water molecules are present for every formula unit of sodium carbonate. Find the mass percentage of water in sodium carbonate decahydrate, Na2C03 lOH O, which has a molar mass of286.19 g/mol. 

The mass of water per mole of sodium carbonate decahydrate must first be found. This value is then divided by the mass of one mole of Na2CO3 10H2O. 

One mole ofNa2CO3 10H2O contains 10 mol H2O. As noted earlier, the molar mass of H2O is 18.02 g/mol. The mass of 10 mol H2O is calculated as follows. 

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