Step 1. Make use of the chemical formula to determine the number of atoms of each element in the compound.
Step 2. Multiply the atomic weight of each element with its number of atoms present in the compound.
Step 3. Add up all and assign unit as grams/mole.
Example. 1 What is the molar mass of sodium carbonate, Na2CO3 ?
Solution Since sodium carbonate contains two atoms sodium, one atom of carbon and three atoms of oxygen. The molecular weight would be
Na : 2 x 23.0 = 46
C : 1 x 12.0 = 12
O : 3 x 16 = 48
When we add up the total values i.e, 46 + 12 + 48 = 106
Therefore, the molar mass of Na2CO3 is 106 g/mole.
Example 2.Identify the molar mass of calcium nitrate, Ca(NO3)2 ?
Solution: Since calcium nitrate contains one atom of calcium, two atoms of nitrogen and six atoms of oxygen.
Ca: 1 x 40.1 = 40.1
N: 2 x 14.0 = 28
O: 6 x 16.0 = 96
If we add all, 40.1 + 14 + 16 = 164.1
Therefore, the molar mass of Ca(NO3)2 is 164.1 g/mol.
Note that the subscript two after the parentheses specifies that there are 2 nitrate ions (NO3-). Multiply the number of atoms with the subscripts outside the parenthesis. Subscripts outside the ( ) affect only the atoms inside the ( ) and not the Ca ion.
To solve more examples and practice questions on molar mass, please visit Byju’s.com