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 = _____ %