how to find mass percent

Ruby Gemstone

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 mass_A is the mass of the part you are interested in and mass_TOTAL 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

mass_Al = 2⋅26.98 g/mol = 53.96 g/mol

There are three atoms of oxygen:

mass_O = 3⋅16.00 g/mol = 48.00 g/mol

Add these together to get the total mass of Al₂O₃

mass_TOTAL = mass_Al + mass_O
mass_TOTAL = 53.96 g/mol + 48.00 g/mol
mass_TOTAL = 101.96 g/mol

Now we have all the information we need to find the mass % of each element. Let's start with aluminum.

$mass percent of aluminum$

mass % Al = 0.53 ⋅ 100
mass % Al = 53%

Find mass % of oxygen.

$mass percent of oxygen$

mass % O = 0.47 ⋅ 100
mass % O = 47%

Answer

In aluminum oxide, Al₂O₃, aluminum accounts for 53% of the total mass and oxygen is 47% of the total mass. Even though the two elements make up nearly equal parts of the molecule, there is more aluminum by mass in one molecule of aluminum oxide.

Remember to check your answers in mass percent by adding up the individual parts together to see if you have 100%. For our example, 53% + 47% = 100%. Our answer checks out.