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.
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?
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%
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.