Mass Percent Composition Problem

Mass percent composition expresses the relative amount of each element in a compound in terms of mass.
Mass percent composition expresses the relative amount of each element in a compound in terms of mass. Astrakan Images / Getty Images

This is a worked example problem showing how to calculate mass percent composition. Percent composition indicates the relative amounts of each element in a compound. For each element:

% mass = (mass of element in 1 mole of the compound) / (molar mass of the compound) x 100%

The units of mass are typically grams. The molar mass is the sum of the masses of all the atoms in one mole of the compound. The sum all the mass percentages should add up to 100%.

Watch for rounding errors in the last significant figure to make sure all the percentages add up.

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

Percent Composition of Water

Another simple example is finding the mass percent composition of the elements in water, H2O.

First, find the molar mass of water by adding up the atomic masses of the elements. Use values from the periodic table:

H is 1.01 grams per mole
O is 16.00 grams per mole

Get the molar mass by adding up all the masses of elements in the compound. The subscript after the hydrogen (H) indicates there are two atoms of hydrogen. There is no subscript after oxygen (O), which means only one atomĀ is present.

molar mass = (2 x 1.01) + 16.00
molar mass = 18.02

Now, divide the mass of each element by the total mass to get the mass percentages:

mass % H = (2 x 1.01) / 18.02 x 100%
mass % H = 11.19%

mass % O = 16.00 / 18.02
mass % O = 88.81%

The mass percentages of hydrogen and oxygen add up to 100%.