Molality
Definition
A measure of solution concentration defined as moles of solute per kilogram of solvent (mol/kg or m).
Why is Molality Important?
Molality is an essential chemistry concept used in laboratories, pharmaceutical development, environmental science, and industrial processes. Understanding this concept is critical for accurate chemical calculations, safe laboratory practices, and optimizing reactions.
Our chemistry calculators provide instant, accurate results for complex conversions and calculations, making lab work more efficient and reducing the risk of errors in critical measurements.
What is Molality?
Molality (m) is a measure of solution concentration defined as the number of moles of solute per kilogram of solvent: m = mol solute / kg solvent. Unlike molarity, molality does not change with temperature because mass (unlike volume) is independent of temperature.
Molality vs Molarity
| Property | Molarity (M) | Molality (m) |
|---|---|---|
| Definition | mol solute / L solution | mol solute / kg solvent |
| Units | mol/L (M) | mol/kg (m) |
| Denominator | Total solution volume | Solvent mass only |
| Temperature dependent? | Yes (volume changes) | No (mass doesn't change) |
| Easier to prepare? | Yes | Slightly harder (must weigh solvent) |
Colligative Properties (Where Molality Matters)
| Property | Formula | Constant for Water |
|---|---|---|
| Boiling point elevation | ΔTb = Kb × m × i | Kb = 0.512 °C/m |
| Freezing point depression | ΔTf = Kf × m × i | Kf = 1.86 °C/m |
Example: Road salt (NaCl) at 1m concentration depresses water's freezing point by 1.86 × 1 × 2 = 3.72°C (6.7°F) because NaCl dissociates into 2 ions (i=2).