How to Calculate Molar Mass

The fact that you are checking out this article just now, means that you are probably studying for a general chemistry course and have assignments to be taken care of! I have been there so I understand that it can be tedious at times, when you do not have a clear grasp of underlying principles of a chemistry problem.

In chemistry, we deal with various types of chemical reactions. To quantify the reactants and the products, one needs to ensure that they are used in definite proportions. To ensure that we get reproducible results, definite quantities of reactants and products need to be measured.

What is Molar Mass?

Under the SI system of units, a mole is defined as the amount of a substance (a chemical element or compound) that contains as many elementary entities (molecules, atoms or ions), as there are atoms in 12 gm of Carbon-12. A mole contains exactly 6.023 x 10²³ (Avogadro's number) elementary entities (atoms, molecules or ions) of that substance.

So, think of a mole of a substance as an amount that contains exactly 6.023 x 10²³ molecules and atoms. So, molar mass of a substance is the mass of one mole of that substance. Let us see the calculation method. Keep a periodic table with atomic mass in front of you, along with a calculator. Also, while calculating, you need to know the molecular formula of the concerned compound.

Calculation For Elements

Molar mass of an element is the atomic weight of the element expressed in grams/mol. See which element's mass is to be calculated and note its standard atomic weight. Multiply it by 1 g/mol, which is the standard constant. The product of the standard atomic weight of the element with 1 g/mol will give you the molar mass.

For example, if you need to compute the molar mass of Carbon, then you need to note down its atomic weight first, which is 12.01. Next multiply it with 1 gm/mol and the product, 12.01 gm/mol is the molar mass of Carbon.

Calculation For Compounds

The molar mass of a compound is a sum of the molar masses of atoms, that make the compound. Here is the calculation procedure. Get the molecular formula of the compound. Note down the different atomic elements that are contained in the molecular formula, along with the number of atoms of each element. Note down the standard atomic weights for each element. Then after multiplying the standard atomic weights of each element by the number of atoms of each element, sum all the products up. Next multiply the sum by the constant - 1 gm/mol and you have what you need.

Let us see the calculation for water, whose molecular formula is H₂O. Hydrogen has two atoms while oxygen has only one. So molar mass of water will be:

Molar mass of Water (H₂O) = (2 x 1.00794 + 1 x 15.9994 ) x 1 gm/mol = 18.015 gm/mol

Calculating this parameter is purely a plug-in problem once you understand the concept. Just put in the numbers and get the answer.