# 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 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

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