Molality (m) | Concentration Terms Class 11

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.