1. J OURNAL #72 How many moles of compound are there in 6.00g of ammonium sulfate? How many molecules are there in 20.0g sulfuric acid?

2. T ODAY WE WILL C ALCULATE P ERCENTAGE C OMPOSITION

3. P ERCENTAGE C OMPOSITION It is useful to know the percentage by mass of a particular element in a chemical compound This is called percentage composition. For example: suppose the compound potassium chlorate were to be used as a source of oxygen. It would be helpful to know the percentage of oxygen in the compound.

4. P ERCENTAGE C OMPOSITION To find the mass percentage of an element in a compound, you divide the mass of the element in the sample by the total mass of the sample. Then, multiply this value by 100. Mass of element in sample of compound Mass of sample of compound X 100 = % element in compound

5. E XAMPLES : Calculate the percentage composition of copper (I) sulfide. Calculate the percentage composition of sodium nitrate

6. E XAMPLES : Calculate the percentage composition of lead (II) chloride Calculate the percentage composition of barium nitrate

7. E XAMPLE : 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, NaCO3 ● 10H20, which has a molar mass of 286.19g/mol.