Define molarity, molality and mole fraction. Differentiate between them.

📘 Molarity, Molality and Mole Fraction

The concentration of a solution can be expressed in different ways. Three important methods are Molarity, Molality and Mole Fraction.

1️⃣ Molarity (M)

Molarity is defined as the number of moles of solute present in one litre of solution.

M = Moles of Solute ÷ Volume of Solution in Litres

Where:

• M = Molarity
• n = moles of solute
• V = volume of solution in litres

M = n / V

Unit: mol L⁻¹ or M

🎬 Molarity Visualization

Molarity depends on the volume of the solution.

S S S

1 L Solution → Solute particles distributed throughout solution

2️⃣ Molality (m)

Molality is defined as the number of moles of solute present in one kilogram of solvent.

m = Moles of Solute ÷ Mass of Solvent in kg

m = n / W

Where:

• m = Molality
• n = moles of solute
• W = mass of solvent in kg

Unit: mol kg⁻¹ or m

🎬 Molality Visualization

Molality depends on the mass of solvent.

S S

1 kg Solvent → Solute particles

3️⃣ Mole Fraction (X)

Mole fraction of a component is the ratio of the number of moles of that component to the total number of moles of all components present in the solution.

X_A = n_A ÷ (n_A + n_B + ...)

For a two-component solution:

X_A = n_A / (n_A + n_B)

X_A + X_B = 1

Unit: No unit

🧮 Suitable Example

Suppose a solution contains:

2 mol Water + 3 mol Ethanol

Total number of moles:

2 + 3 = 5 mol

Mole fraction of water:

X_H₂O = 2 / 5 = 0.4

Mole fraction of ethanol:

X_Ethanol = 3 / 5 = 0.6

0.4 + 0.6 = 1

📊 Difference Between Molarity, Molality and Mole Fraction

Property	Molarity	Molality	Mole Fraction
Definition	Moles of solute per litre of solution	Moles of solute per kg of solvent	Moles of component ÷ total moles
Formula	M = n/V	m = n/W	X = n/Σn
Based on	Volume of solution	Mass of solvent	Number of moles
Unit	mol L⁻¹	mol kg⁻¹	No unit
Temperature effect	Changes with temperature	Independent of temperature	Independent of temperature

⭐ Most Important Difference

Molarity uses the volume of solution, whereas molality uses the mass of solvent.

Molarity → Volume of Solution

Molality → Mass of Solvent

Mole Fraction → Ratio of Moles

🎯 Quick Revision

Molarity: moles of solute / litre of solution

Molality: moles of solute / kg of solvent

Mole Fraction: moles of component / total moles

✅ Remember:

Molarity → Solution Volume
Molality → Solvent Mass
Mole Fraction → Mole Ratio