Mass Percent Composition Problem

Chemistry involves mixing one substance with another and observing the results. To replicate the results, it's important to measure amounts carefully and record them. Mass percent is one form of measurement used in chemistry; understanding mass percent is important for accurately reporting in chemistry labs.

What Is Mass Percent?

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 any unit of measure is acceptable 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

In this procedure, we will work out the answer to the question "What are the mass percentages of carbon and oxygen in carbon dioxide, CO₂?"

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 CO_2.

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

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 %

Solution

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.