Percentage Composition

It is exactly what it sounds like. Percentage Composition.
This class we learned about Percentage Composition and it is quite simple.

Here is an example

What is the percentage composition of NaCl?

First of all you must calcuate the total Molar Mass(MM) of this compound

Total MM of NaCl: 23.0 + 35.5 = 58.5g/mol

MM of Na = 23.0 g/mol
MM of Cl  = 35.5 g/mol

From these numbers you calculate the percent composition of Na and Cl. (Round to 1 decimal place if the question does not give any numbers)

% of Na = 23.0g/mol
                 ------        x 100        =        39.3%                                % composition = mass of element
                 58.5g/mol                                                                                                  ---------------- x100
                                                                                                                                  mass of compunt
% of Cl  = 35.5g/mol
                 ------      x 100          =       60.7%
                 58.5g/mol

From these two numbers, the %  composition should add up to 100%.
Here is a video that explains another example of percent composition.



Here are the five questions and solutions we had to make.

1) What is the percent composition of Potassium carbonate? (K ₂ CO₃ )

Total MM of K2CO3 is :

K x 2 = 39.1 x 2 = 78.2 g/mol

C x  1 = 12.0 x 1 = 12.0 g/mol

O x 3 = 16.0 x 3 = 48.0g/mol

                          =  138.2g/mol
% of K = 78.2g/mol  /  138.2g/mol = 56.6%
% of C = 12.0g/mol  /  138.2g/mol = 8.7%
% of O = 48.0g/mol  /  138.2g/mol = 34.7%

2. Sodium Bicarbonate (sodium hydrogen carbonate) is basically baking soda. Its formula is NaHCO₃. Find the mass percentages (mass %) of Na, H, C, and O in sodium hydrogen carbonate.

Total MM = 84.0 g/mol

mass % Na = 23.0 g / 84.0 g x 100 = 27.4 %
mass % H = 1.0 g / 84.0 g x 100 = 1.2 %
mass % C = 12.0 g / 84.0 g x 100 = 14.3 %
mass % 3 * O = 48.0 g / 84.0 g x 100 = 57.1 %

3. Cetylpyridinium chloride, a common compound found in mouthwash, has a formula of C₂₁H₃₈NCl.
What is the percent composition?


21 C = 252 u
38 H = 38.0 u
1 N = 14.0 u
1 Cl = 35.5 u


Total MM = 339.5 g/mol


mass % 21 C = (252/339.5)100 = 74.2%
mass % 38 H = (38.0/339.5)100 = 11.2%
mass % 1 N = (14.0/339.5)100 = 4.12%
mass % 1 Cl = (35.5/339.5)100 = 10.5%

4. C₉H₁₁N₂O₄S is penicillin. Find the percentage composition for each Element.

Atomic mass

C9 = 108g/mol

H 11= 11g/mol

N2=28g/mol

O2 = 64g/mol

S = 32g/mol

Total MM = 234g/mol

% of C = 108/234 = 44%

% of H = 11/234 = 5%

% of N = 28/234 = 11%

% of O =64/234 = 27%

% of S = 32/234 = 13%