How to Calculate the Molar Mass of a Compound

Add together the atomic weights to find the molar mass of a compound.

Molar mass indicates the total of all of the masses of the elements that make up a compound. The molar mass indicates the number of grams per one mole of that compound, according to the University of Memphis. Knowing the grams per mole allows chemists to convert a compound from moles into grams.

Things You'll Need

• Periodic table

Instructions

1. Look up the atomic mass of all of the elements in the compound using a periodic table. For instance, note that on a periodic table, the atomic weight of carbon equals 12.01 grams and oxygen's atomic weight equals 16.00 grams for the compound of CO2.

2. Note the number of each element in the compound and multiply each of the atomic weight by the number of atoms of that element in the compound. For example, CO2 contains one carbon and two oxygen atoms. Multiply carbon's atomic weight (12.01 grams) by the number of carbon atoms in the compound (1) to find: 1 x 12.01 = 12.01 grams. Repeat the process with oxygen to find: 2 x 16.00 = 32.00 grams.

3. Add all of the weights of the elements in the compound together to find the molar mass. For the example, add 12.01 + 32.00 = 44.01 grams/mole (g/mol) for the molar mass of CO2.

References

• ASUB: Molar Mass or Gram Mass Formula
• Carnegie Mellon University: How to Calculate Molar Mass
• Frostburg: General Chemistry Online - Counting Molecules By Weighing
• University of Memphis: The Mole and Molar Mass

Resources

• Web Elements: Periodic Table