Differentiate between molarity and molality. Write their mathematical expressions and explain the effect of temperature on each.

📘 Molarity vs Molality

🔵 1. Molarity

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

M = n / V

Where:

• M = Molarity in mol L⁻¹
• n = Number of moles of solute
• V = Volume of solution in litres

🟠 2. Molality

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

m = n / W

Where:

• m = Molality in mol kg⁻¹
• n = Number of moles of solute
• W = Mass of solvent in kilograms

🎯 Concentration Concept

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🔗 Difference Between Molarity and Molality

Basis	Molarity	Molality
Definition	Moles of solute per litre of solution	Moles of solute per kg of solvent
Formula	M = n/V	m = n/W
Volume / Mass	Volume of solution	Mass of solvent
Unit	mol L⁻¹	mol kg⁻¹
Temperature Effect	Changes with temperature	Generally independent of temperature

🌡️ Effect of Temperature

Molarity and Temperature

Molarity depends on the volume of solution. Volume can change with temperature.

Temperature ↑ → Volume ↑ → Molarity ↓

Thus, molarity is temperature dependent.

Molality and Temperature

Molality depends on the mass of solvent. Mass does not appreciably change with temperature.

Temperature Change → Mass of Solvent ≈ Constant → Molality ≈ Constant

Therefore, molality is considered temperature independent.

🎬 Temperature Effect Animation

Molarity

Solute Solute
🌡️ ↑
Volume changes

Molality

Solute Solute
🌡️ ↑
Mass remains constant

🌡️ Temperature ↑

Molarity → Changes
Molality → Remains approximately constant

📝 Simple Example

Suppose 1 mole of NaCl is dissolved to prepare a solution of 2 L volume.

M = 1 / 2

M = 0.5 mol L⁻¹

If temperature changes and the solution volume expands, the molarity changes.

However, if 1 mole NaCl is dissolved in 2 kg of solvent:

m = 1 / 2

m = 0.5 mol kg⁻¹

The mass of the solvent remains essentially unchanged with temperature, so the molality remains approximately constant.

✅ Final Answer

Molarity: M = moles of solute / volume of solution in L

Molality: m = moles of solute / mass of solvent in kg

🌡️ Molarity is temperature dependent.
🌡️ Molality is approximately temperature independent.

🎯 Quick Revision

Molarity → Volume of Solution → Temperature Dependent

Molality → Mass of Solvent → Temperature Independent