Mass Percentage Definition and Example

Understand Mass Percentage in Chemistry

Mass percentage represents the concentration of a sample.
Mass percentage represents the concentration of a sample. Glow Images, Inc / Getty Images

Mass Percentage Definition

Mass percentage is one way of representing the concentration of an element in a compound or a component in a mixture. Mass percentage is calculated as the mass of a component divided by the total mass of the mixture, multiplied by 100%.

Also Known As: mass percent, (w/w)%

Mass Percentage Formula

Mass percent is the mass of the element or solute divided by the mass of the compound or solute.

The result is multiplied by 100 to give a percent.

The formula for the amount of an element in a compound is:

mass percent = (mass of element in 1 mole of compound / mass of 1 mole of compound) x 100

The formula for a solution is:

mass percent = (grams of solute / grams of solute plus solvent) x 100

or

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

The final answer is given as %.

Mass Percentage Examples

Example 1: Ordinary bleach is 5.25% NaOCl by mass, which means each 100 g of bleach contains 5.25 g NaOCl.

Example 2: Find the mass percentage of 6 g sodium hydroxide dissolved in 50 g of water. (Note: since the density of water is nearly 1, this type of question often gives the volume of water in milliliters.)

First find the total mass of the solution:

total mass = 6 g sodium hydroxide + 50 g water
total mass = 56 g

Now, you can find the mass percentage of the sodium hydroxide using the formula:

mass percent = (grams of solute / grams of solution) x 100
mass percent = (6 g NaOH / 56 g solution) x 100
mass percent = (0.1074) x 100
answer = 10.74% NaOH

Example 3: Find the masses of sodium chloride and water required to obtain 175 g of a 15% solution.

This problem is a bit different because it gives you the mass percentage and asks you to then find how much solute and solvent are needed to yield a total mass of 175 grams.

Start with the usual equation and fill in the given information:

mass percent = (grams solute / grams solution) x 100
15% = (x grams sodium chloride / 175 g total) x 100

Solving for x will give you the amount of NaCl:

x = 15 x 175 / 100
x = 26.25 grams NaCl

So, now you know how much salt is needed. The solution consists of the sum of the amount of salt and water. Simply subtract the mass of salt from the solution to obtain the mass of water that is required:

mass of water = total mass - mass of salt
mass of water = 175 g - 26.25 g
mass of water = 147.75 g

Example 4: What is the mass percent of hydrogen in water?

First, you need the formula for water, which is H2O. Next you look up the mass for 1 mole of hydrogen and oxygen (the atomic masses) using a periodic table.

hydrogen mass = 1.008 grams per mole
oxygen mass = 16.00 grams per mole

Next, you use the mass percentage formula. The key to performing the calculation correctly is to note there are 2 atoms of hydrogen in each water molecule. So, in 1 mole of water there are 2 x 1.008 grams of hydrogen. The total mass of the compound is the sum of the mass of the two hydrogen atoms and one oxygen atom.

mass percent = (mass of element in 1 mole of compound / mass of 1 mole of compound) x 100
mass percent hydrogen = [(2 x 1.008) / (2 x 1.008 + 16.00)] x 100
mass percent hydrogen = (2.016 / 18.016) x 100
mass percentage hydrogen = 11.19%