Molality Formula
Molality (m) equals moles of solute divided by kilograms of solvent.
Used for colligative properties like boiling point elevation.
The Formula
Molality measures the concentration of a solution by comparing moles of solute to the mass of solvent (not the total solution volume). Unlike molarity, molality does not change with temperature because mass does not expand or contract.
This makes molality especially useful for colligative property calculations like boiling point elevation and freezing point depression.
Variables
| Symbol | Meaning |
|---|---|
| m | Molality (in mol/kg, sometimes written as "molal") |
| moles of solute | Amount of dissolved substance (in mol) |
| kg of solvent | Mass of the solvent only, not the total solution (in kg) |
Molality vs Molarity
| Property | Molality (m) | Molarity (M) |
|---|---|---|
| Definition | mol solute / kg solvent | mol solute / L solution |
| Temperature dependent? | No | Yes |
| Uses mass or volume? | Mass of solvent | Volume of solution |
| Best for | Colligative properties | Volumetric analysis |
Example 1
You dissolve 58.44 g of NaCl (1 mole) in 500 g of water. What is the molality?
Moles of NaCl = 58.44 g / 58.44 g/mol = 1.00 mol
Mass of solvent = 500 g = 0.500 kg
m = 1.00 / 0.500
m = 2.00 mol/kg (2.00 molal)
Example 2
A solution of glucose (C₆H₁₂O₆, molar mass 180.16 g/mol) is prepared by dissolving 36 g of glucose in 400 g of water. What is the molality?
Moles of glucose = 36 / 180.16 = 0.200 mol
Mass of solvent = 400 g = 0.400 kg
m = 0.200 / 0.400
m = 0.500 mol/kg (0.500 molal)
When to Use It
Molality is preferred over molarity in several situations.
- Boiling point elevation calculations (ΔT_b = K_b × m)
- Freezing point depression calculations (ΔT_f = K_f × m)
- Osmotic pressure problems in some formulations
- Any experiment where temperature changes significantly (because molality is temperature-independent)
- Precise thermodynamic calculations