Molality ($m$)
The Temperature-Independent Concentration Term | Class 11 Chemistry
1. Definition & Formula
Molality ($m$) is defined as the number of moles of solute present in 1 kg (1000 g) of the solvent.
$$ m = \frac{\text{Moles of Solute ($n$)}}{\text{Mass of Solvent ($W$) in kg}} $$
Expanded Formula:
$$ m = \frac{w_2 \times 1000}{M_2 \times w_1} $$Where:
$w_2$ = Mass of Solute (g)
$M_2$ = Molar Mass of Solute (g/mol)
$w_1$ = Mass of Solvent (g)
Unit: $mol/kg$ or Molal ($m$).
2. Why is Molality Important?
Unlike Molarity ($M$), which involves Volume, Molality involves Mass.
Temperature Independence: Since mass does not change with temperature (unlike volume which expands/contracts), Molality does not change with temperature.
This makes Molality the preferred unit for calculating Colligative Properties (Boiling Point Elevation, Freezing Point Depression).
3. Relation between Molality ($m$) and Molarity ($M$)
If the Density of the solution is $d$ (g/mL) and Molar Mass of solute is $M_{solute}$:
$$ m = \frac{1000 \times M}{1000d - M \times M_{solute}} $$
This formula relates moles per liter to moles per kg solvent.
4. Molarity vs Molality
| Feature | Molarity ($M$) | Molality ($m$) |
|---|---|---|
| Basis | Volume of Solution | Mass of Solvent |
| Unit | mol / L | mol / kg |
| Temp Effect | Changes with Temp | Constant |
| Precision | Less precise for T-changes | More precise |
Practice Quiz
Test your numerical skills on Molality.
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