Mole Concept and Gram Equivalent Mass with Its important Numericals

Mole Concept, Molar Mass, Molar Volume, and Gram Equivalent Mass

Mole Concept

The mole concept is a method used to calculate the amount of substance present. One mole of any substance contains Avogadro’s number of particles, which is 6.022 × 10²³ particles. The mole is the bridge between the microscopic world of atoms/molecules and the macroscopic world that we can observe.

Example: 1 mole of carbon atoms contains 6.022 × 10²³ carbon atoms.

Numerical Problem:

Problem: How many atoms are there in 2 moles of sodium (Na)?

Solution:
Number of atoms = Moles × Avogadro’s number
Number of atoms = 2 × 6.022 × 10²³ atoms = 1.2044 × 10²⁴ atoms

Molar Mass

The molar mass of a substance is the mass of one mole of that substance. It is expressed in grams per mole (g/mol). For elements, the molar mass is numerically equal to its atomic mass.

Example: The molar mass of water (H₂O) is calculated as follows:
Molar mass of H₂O = (2 × Atomic mass of H) + (1 × Atomic mass of O)
= (2 × 1 g/mol) + (16 g/mol)
= 18 g/mol

Numerical Problem:

Problem: Calculate the molar mass of NaCl.

Solution:
Molar mass of NaCl = Atomic mass of Na + Atomic mass of Cl
= 23 g/mol + 35.5 g/mol
= 58.5 g/mol

Molar Volume

Molar volume is the volume occupied by one mole of any substance at standard temperature and pressure (STP). For gases, the molar volume is 22.4 L at STP.

Example: 1 mole of any ideal gas at STP occupies 22.4 L.

Numerical Problem:

Problem: What is the volume occupied by 3 moles of carbon dioxide (CO₂) gas at STP?

Solution:
Volume of gas = Moles × Molar volume
Volume of CO₂ = 3 × 22.4 L
= 67.2 L

Gram Equivalent Mass

Gram equivalent mass is the mass of a substance that reacts with or supplies 1 mole of hydrogen ions (H⁺) in a reaction. It is given by the formula:

Equivalent mass = ^{Molar mass} / _Valency

Example: The equivalent mass of sulfuric acid (H₂SO₄) is calculated as:
Equivalent mass of H₂SO₄ = ^{98 g/mol} / ₂
= 49 g/mol

Numerical Problem:

Problem: Calculate the gram equivalent mass of calcium hydroxide [Ca(OH)₂].

Solution:
Equivalent mass = ^{Molar mass} / _Valency
Molar mass of Ca(OH)₂ = 40 + (2 × 16) + (2 × 1)
= 40 + 32 + 2
= 74 g/mol
Equivalent mass = 74 g/mol / 2
= 37 g/mol