A mole is a counting unit referring to the number of atoms, molecules, ions, or formula units that exist in any given chemical compound. For comparison, the other counting units are a pair (2) and dozen (12). Any compound has, one mole equals Avogadro's number, which is 6.02214076 × 1023 mol, of molecules or formula units. The molar mass of a compound is the mass of 1 mole of that particular substance and the number of grams per mole of a compound. Or, in other words, the molar mass is the total mass of all the atoms in grams that make a mole of a particular molecule. Hence, grams/mole is the units of molar mass. 

The molar mass of a chemical can be determined in more than one ways:

• Step 1: Calculate the number of atoms of each element by the formula to obtain its chemical formula.

• Step 2: Multiply the atomic weight of the element with its number of atoms present in the compound.

• Step 3: Sum them and assign the unit as grams/mole.

Solved Example

Example 1:  What is the molar mass of sodium carbonate,?

Solution: As 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    is 106 g/mole.

 

 

 

Example 2: Give the molar mass of calcium nitrate?

Solution: As calcium nitrate contains one atom of calcium, two atoms of nitrogen and six atoms of oxygen.

 Ca: 1 x 40.1 = 40

  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 i s 164.1 g/mol.

 

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