Number of Moles Formula with Solved Example

Number of moles formula

Small description- The number of moles defines the amount of a substance in a chemical equation. The short form of moles is “mol”. 

Moles: 

A mole represents the number of chemical elements in an equation. A mole contains a very large number of particles. The value of one mole of any substance is equal to the Avogadro number. 

Avogadro number = 6.023 × 10²³. 

The Avogadro number is used to measure the products in any chemical reaction. 

6.023 × 10²³ = 1 mol of atoms, molecules or particles 

The Number Of Moles Can Be Calculated As: 

Number of moles = Mass of substance ÷ mass of one mole 

Benefits Of The Mole Concept: 

The unit of the mole is very significant in chemistry. It forms the base of stoichiometry and it is the best for representing the amounts of reactants and products consumed and formed during a chemical reaction. 

All the chemical reactions can be expressed in moles. 

For example- 

2H₂ + O₂ -> 2H₂O 

In the above equation, 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water. 

The value of a mole can also be used to express the concentration of units in a chemical equation. L⁻¹ can be used to express mole per litre or molecular weight. 

Solved Examples: 

Question- Calculate the number of moles in 0.325 grams of barium hydroxide. 

Answer- Mass of barium hydroxide= 0.325 gram 

The molar mass of barium hydroxide= 171 gram 

The number of moles can be calculated as = 

Mass of substance ÷ Mass of 1 mole 

By putting in the values:

Number of moles = 0.325 ÷ 171 

= 0.00190 mol 

Question- Calculate the number of moles in 190 grams of MnO₂. 

Answer- Mass of MnO₂= 190 gram 

Mass of 1 mole of MnO₂= 86.94 g

The number of moles is calculated as: 

Mass of Substance ÷ Mass of 1 mole 

By putting in the values: 

Number of moles = 190 ÷ 86.94 

Number of moles = 2.184 mol 

Question- Calculate the number of moles in 25.0 grams of water. 

Answer- The number of moles is calculated as: 

Mass of substance ÷ Mass of 1 mole 

Molar mass of water = 18.0 g 

Mass of substance = 25.0 g 

Number of moles = 25 ÷ 18 

= 1.39 mol