Percent Composition

PERCENT COMPOSITION:  the percentage by mass of each element in a compound

FORMULA FOR % COMPOSITION:
% composition = mass of element in compound _{x 100}
molar mass of compound

EXAMPLES:
Q1.  Find the % composition of copper (I) sulfide, Cu₂S.
A1.  Finding % composition means that you have to find the % of each element in the cmpd.                    
molar mass of Cu₂S =
            Cu:  __ x _____ = 
            S:    __ x _____ = 
              MM of Cu₂S = 
                % Cu =    ______ _{x 100 =}  

                % S =    ______  _{x 100 =} 

A1 = Cu:  _____ % and S: _____ %
            * For % comp. problems, you can round to 3 SF's. *

Q2.  Find the percent of oxygen in calcium phosphate, Ca₃(PO₄)₂.
A2.  molar mass of Ca₃(PO₄)₂ =
        Ca:  __ x _____ = 
        P:    __ x _____ = 
        O:   __ x _____ = 
        MM of Ca₃(PO₄)₂ = 
        % O =   ______  _{x 100 =}  

A2 = _____ %

Q3.  Find the percent of water in sodium carbonate decahydrate, Na₂CO₃ ^. 10 H₂O.
A3.  (The ^. 10 H₂O means that there are 10 water molecules attached to one sodium carbonate. To figure out the molar mass of this compound, find the molar mass of Na₂CO₃, the molar mass of water times 10, then add those together.)
molar mass of Na₂CO₃:
        Na:  __ x _____ = 
        C:    __ x _____ = 
        O:    __ x _____ = 
        MM of Na₂CO₃ = 
molar mass of H₂O:
     H:  __ x _____ = 
     O:  __ x _____ = 
     MM of H₂O = 
molar mass of Na₂CO₃ ^. 10 H₂O = _____ + (10 x _____) =
             % H₂O =   _______ x 100 = 

A3 = _____ %

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