Percent Composition

Percent composition is the percentage by mass of a "species" in a chemical formula.  You are trying to find the percentage of how much one element is in a compound.  You can find this by using the molar mass of each compound.
**(sometimes the percentage may not add up to 100% because of the rounding.  If the percent is somewhere near 99.9% its ok.)**

Ex.1 What is the percentage composition of NaCl?
First we need the following:
Total Molar Mass of NaCl: 58.5g/mol
Molar Mass of Na: 23.0 g/mol
Molar Mass of Cl: 35.5 g/mol
Then we do the calculations like so:
% of Na = 23    g/mol   x  100% = 39.3%
                 58.5 g/mol

% of Cl = 35.5 g/mol   x   100% = 60.7%
                58.5 g/mol
Total percentage: 100%


Ex.2 What is the percentage composition of ZnSO3?
Total Molar Mass of ZnSO3: 145.5 g/mol
Molar Mass of Zn: 65.4 g/mol
Molar Mass of S: 32.1 g/mol
Molar Mass of O: 16.0 x 3 = 48.0 g/mol

% of Zn = 65.4  g/mol  x  100% = 44.9 %
                145.5 g/mol

% of S = 32.1   g/mol  x  100% = 22.1%
               145.5 g/mol

% of O = 48.0    g/mol  x  100% = 33.0%
                145.5 g/mol
Total percentage: 100.0%

Ex. 3 What is the percentage composition of Zn3(PO4)2?
Total Molar Mass of Zn3(PO4)2: 386.2 g/mol
Molar Mass of Zn3: 65.4 x 3 = 196.2 g/mol
Molar Mass of P: 31.0 x 2 = 62.0 g/mol
Molar Mass of O: 16.0 x 8 = 128.0 g/mol

% of Zn = 196.2 g/mol  x  100% = 50.8%
                 386.2 g/mol

% of P = 62.0    g/mol    x  100% = 16.1%
               386.2 g/mol         

% of O = 128.0 g/mol  x  100% = 33.1%
                386.2 g/mol
Total percentage: 100%

Ex. 4 If a compound contains 54 grams of Al, 36 grams of C, 56 grams of N, some amount of O, and has a total molar mass of 226 grams, what is the percentage composition? 
First, we need to find the molar mass of the amount of O.  To do this we do:
226 - 54 - 36 - 56 = 80  <--- this is the molar mass of O.

% of Al = 54.0   g/mol  x  100% = 23.9%
                226.0 g/mol

% of C = 36.0   g/mol  x  100% = 15.9%
               226.0 g/mol

% of N = 56.0   g/mol  x  100% = 24.8%
               226.0 g/mol

% of O = 80.0    g/mol  x 100% = 35.4%
                226.0 g/mol 
Total percentage: 100%

Ex. 5 If a compound contains 21.6 grams of B, 57.0 grams of F, 80.8 grams of Ne, some amount of He, and has a total molar mass of 179.4, what is the percentage composition? 
What is the molar mass of He?
179.4 - 21.6 - 57.0 - 80.8 = 20 <--- this is the molar mass of He. 

% of B = 21.6   g/mol  x  100% = 12.0%
               179.4 g/mol

% of F = 57.0   g/mol  x  100% = 31.8%
               179.4 g/mol

% of Ne = 80.8   g/mol  x  100% = 45.0%
                 179.4 g/mol

% of He = 20.0   g/mol  x  100% = 11.1%
                 179.4 g/mol
Total percentage: 99.9%
(In this case, the total percentage was not 100% but was 99.9%.  It is just a 0.1% difference and not that big of a deal.  This is the result due to rounding.)

Video similar to Ex 2 & 3:



Video almost similar to Ex. 4 & 5