The mole is a counting unit to specify the number of atoms, molecules, ions or formula units in a particular chemical compound. It is similar to other counting units like a pair (2) and a dozen (12). One mole of a compound contains Avogadro’s number (6.02214076 × 10²³ mol) of molecules or formula units. The molar mass of a compound defines the mass of 1 mole of that particular substance and number of grams per mole of a compound.  In other words, the molar mass is the total mass of all the atoms in grams that make a mole of a particular molecule. Therefore, the units of molar mass are grams/mole.How to find the molar mass of a compound?

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.

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