# Mass Percent Example Problem A ruby is a gemstone comprised mostly of aluminum oxide. Its color comes from the addition of chromium atoms to the crystal lattice. Creative Commons License

Mass percent or mass percent composition is a measurement of concentration. It is a measure of the ratio of the mass of one part of a molecule to the mass of the total molecule and expressed as a percentage.

This example problem shows how to determine the mass percent composition of each element of a molecule and determine which element makes up most of the molecule by mass.

Problem

Rubies and sapphires are gemstones where their crystal structure is mostly comprised of aluminum oxide, Al2O3. Find the mass % of aluminum and oxygen. Which element makes up most of the mass of the molecule?

Solution

The formula to calculate mass % is where massA is the mass of the part you are interested in and massTOTAL is the total mass of the molecule.

Using a Periodic Table, we find

mass Al = 26.98 g/mol
mass O = 16.00 g/mol

There are two atoms of aluminum in a Al2O3 molecule, so

massAl = 2⋅26.98 g/mol = 53.96 g/mol

There are three atoms of oxygen:

massO = 3⋅16.00 g/mol = 48.00 g/mol

Add these together to get the total mass of Al2O3

massTOTAL = massAl + massO
massTOTAL = 53.96 g/mol + 48.00 g/mol
massTOTAL = 101.96 g/mol

Now we have all the information we need to find the mass % of each element. Let’s start with aluminum. mass % Al = 0.53 ⋅ 100
mass % Al = 53%

Find mass % of oxygen. mass % O = 0.47 ⋅ 100
mass % O = 47%