2.  New substance produced  Determine chemical composition  Determine composition by mass  Convert mass to % = % composition  Use atomic & molar mass to determine - correct chemical formula

3.  % composition of water  Water = H + O  Results reveal - 2.5 g H reacted = 22.5 g of water  m H = 2.5g  m H2O = 22.5g  m o = (22.5g – 2.5g) = 20.0g  %H = 2.5g/22.5g x 100% = 11.1%  %O = 20.0g/22.5g x 100% = 88.9%

4. Sample Problem  Sodium and chlorine when they are allowed to react - combine to form sodium chloride  In an experiment, 3.45 g of sodium metal reacted with 5.33 g of chlorine gas to give 8.78 g of sodium chloride  Calculate % composition by mass of sodium chloride.

5. Solution  m Na+ =  m Cl- =  m NaCl =  %Na =  %Cl =

6.  Calculate contribution of each element to the total mass  1 st - cal mass of each element + atoms  2 nd - cal total mass of all elements in compound  3 rd - divide mass of each element by total mass = % comp

7. Sample Problem  Determine % composition of sodium phosphate Na 3 PO 4 Solution