Mole Concept – Definition, Formulas, Examples & MCQs

The mole concept is one of the most fundamental topics in chemistry. It provides a way to relate the mass of a substance to the number of particles (atoms, molecules, or ions) it contains. This concept serves as a bridge between the macroscopic and microscopic worlds of chemistry.

Definition of Mole

One mole of any substance is defined as the amount of that substance which contains exactly 6.022 × 10²³ particles (Avogadro’s number). These particles may be atoms, molecules, ions, or electrons depending on the substance.

Important Formulas

• Number of moles (n) = Given mass (m) / Molar mass (M)
• Number of particles (N) = n × Avogadro’s number (N_A)
• Molar volume (at STP) = 22.4 L for 1 mole of gas

Examples

Example 1: Calculate the number of moles in 36 g of water (H₂O).
Molar mass of H₂O = 18 g/mol
n = 36 / 18 = 2 moles

Example 2: Calculate the number of molecules in 44 g of CO₂.
Molar mass of CO₂ = 44 g/mol → 1 mole
Number of molecules = 6.022 × 10²³

Applications of Mole Concept

• Stoichiometric calculations in chemical reactions
• Determining empirical and molecular formulas
• Calculating gas volumes at STP
• Quantitative analysis in titrations

Quick Reference Table

Quantity	Relation
Number of moles (n)	n = m / M
Number of particles (N)	N = n × NA
Volume of gas at STP	V = n × 22.4 L

MCQs on Mole Concept

1. 1 mole of NaCl contains:
   A) 6.022 × 10²³ atoms
   B) 6.022 × 10²³ molecules
   C) 2 × 6.022 × 10²³ ions ✅
   D) 12.044 × 10²³ atoms
2. The molar volume of a gas at STP is:
   A) 22.4 mL
   B) 22.4 L ✅
   C) 224 L
   D) 2.24 L

True or False

• 1 mole of oxygen molecules contains 6.022 × 10²³ O₂ molecules. ✅ True
• Molar mass of a substance is always equal to its atomic mass. ❌ False (for compounds, molar mass = sum of atomic masses)

FAQs

What is 1 mole equal to?

1 mole equals 6.022 × 10²³ particles of a substance.

How many liters does 1 mole of gas occupy at STP?

At STP, 1 mole of gas occupies 22.4 liters.

Wikipedia: Mole (unit)
Khan Academy: Mole Concept