Mass Percent Composition Problem

This is a worked example problem showing how to calculate mass percent composition.

Problem

Bicarbonate of soda (sodium hydrogen carbonate) is used in many commercial preparations. Its formula is NaHCO₃. Find the mass percentages (mass %) of Na, H, C, and O in sodium hydrogen carbonate.

Solution

First, look up the atomic masses for the elements from the Periodic Table. The atomic masses are found to be:

Na is 22.99
H is 1.01
C is 12.01
O is 16.00

Next, determine how many grams of each element are present in one mole of NaHCO₃:

22.99 g (1 mol) of Na
1.01 g (1 mol) of H
12.01 g (1 mol) of C
48.00 g (3 mole x 16.00 gram per mole) of O

The mass of one mole of NaHCO₃ is:

22.99 g + 1.01 g + 12.01 g + 48.00 g = 84.01 g

And the mass percentages of the elements are

mass % Na = 22.99 g / 84.01 g x 100 = 27.36 %
mass % H = 1.01 g / 84.01 g x 100 = 1.20 %
mass % C = 12.01 g / 84.01 g x 100 = 14.30 %
mass % O = 48.00 g / 84.01 g x 100 = 57.14 %

Answer

mass % Na = 27.36 %
mass % H = 1.20 %
mass % C = 14.30 %
mass % O = 57.14 %

When doing mass percent calculations, it is always a good idea to check to make sure your mass percents add up to 100% (helps catch math errors):

27.36 + 14.30 + 1.20 + 57.14 = 100.00