Mass Percent Composition Problem

How to Work Mass Percent Problems in Chemistry

Mass percent expresses concentration of a solute, element, or component of a solution, compound, or mixture.
Mass percent expresses concentration of a solute, element, or component of a solution, compound, or mixture. Glow Images, Inc / Getty Images

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 then multiplied by 100 to get the percent. The formula is:

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

or

mass percent = (mass of solute / mass of solution) x 100%

Usually, mass is expressed in grams, but it honestly doesn't matter as long as you use the same units for both the component or solute mass and the total or solution mass.

Mass percent is also known as percent by weight or w/w%. This worked example problem shows the steps necessary to calculate mass percent composition.

Mass Percent Problem

What are the mass percentages 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. It's a good idea at this point to settle on the number of significant figures you'll be using. 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 %

Answer

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.

Tips for Success Calculating Mass Percent

  • You won't always be given the total mass of a mixture or solution. Often, you'll need to add up the masses. This might not be obvious! You may be given mole fractions or moles and then need to convert to a mass unit.
  • Watch your significant figures!
  • Always make sure the sum of the mass percentages of all components adds up to 100%. If it doesn't, then you need to go back and find your mistake.
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Helmenstine, Anne Marie, Ph.D. "Mass Percent Composition Problem." ThoughtCo, Sep. 7, 2016, thoughtco.com/mass-percent-composition-problem-609566. Helmenstine, Anne Marie, Ph.D. (2016, September 7). Mass Percent Composition Problem. Retrieved from https://www.thoughtco.com/mass-percent-composition-problem-609566 Helmenstine, Anne Marie, Ph.D. "Mass Percent Composition Problem." ThoughtCo. https://www.thoughtco.com/mass-percent-composition-problem-609566 (accessed December 15, 2017).