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Mass Percent Composition Problem

Worked Example Problem for Calculating Mass Percent


Mass percent expresses concentration.

Mass percent expresses concentration.

Ben Mills, Public Domain

Mass percent is a method of expressing the concentration of a substance in a mixture or element in a compound. It is calculated as the mass of the component divided by the total mass of the mixture and multiplied by 100. This worked example problem shows the steps necessary to calculate mass percent composition.

Mass Percent Problem

What are the mass % of carbon and oxygen in carbon dioxide, CO2?

Mass Percent Solution

Step 1: Find the mass of the individual atoms.

Look up the atomic masses for carbon and oxygen from the Periodic Table. The atomic masses are found to be:

C is 12.01 g/mol
O is 16.00 g/mol

Step 2: Find the number of grams of each component make up one mole of CO2.

One mole of CO2 contains 1 mole of carbon atoms and 2 moles of oxygen atoms.

12.01 g (1 mol) of C
32.00 g (2 mole x 16.00 gram per mole) of O

The mass of one mole of CO2 is:

12.01 g + 32.00 g = 44.01 g

Step 3: Find the mass percent of each atom.

mass % = (mass of component/mass of total) x 100

And the mass percentages of the elements are

For carbon:

mass % C = (mass of 1 mol of carbon/mass of 1 mol of CO2)x 100
mass % C = (12.01 g / 44.01 g) x 100
mass % C = 27.29 %

For oxygen:

mass % O = (mass of 1 mol of oxygen/mass of 1 mol of CO2)x 100
mass % O = (32.00 g / 44.01 g) x 100
mass % O = 72.71 %


mass % C = 27.29 %
mass % O = 72.71 %

When doing mass percent calculations, it is always a good idea to check to make sure your mass percents add up to 100%. This will help catch any math errors.

27.29 + 72.71 = 100.00

The answers add up to 100% which is what was expected.

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