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Molality is the number of moles of solute per kilogram of solvent. Unlike molarity, it does not change with temperature, which is why it is used for colligative properties.
This calculator gives the molality and, assuming the solvent is water, the depression in freezing point and elevation in boiling point (Kf = 1.86 K·kg/mol, Kb = 0.512 K·kg/mol), as in Class 12 Solutions.
| Mass of solute | 18 g |
| Molar mass of solute | 180.16 g/mol |
| Mass of solvent | 500 g |
| van 't Hoff factor (i) | 1 |
| Molality (mol/kg) | 0.199827 |
| Moles of solute (mol) | 0.0999134 |
| Freezing point depression in water (°C) | 0.371678 |
| New freezing point of water (°C) | -0.371678 |
| Boiling point elevation in water (°C) | 0.102311 |
| New boiling point of water (°C) | 100.102 |
The molality is 0.199827 mol/kg, so the water freezes at -0.371678 °C.
Try this example →| Mass of solute | 10 g |
| Molar mass of solute | 58.44 g/mol |
| Mass of solvent | 1,000 g |
| van 't Hoff factor (i) | 2 |
| Molality (mol/kg) | 0.171116 |
| Moles of solute (mol) | 0.171116 |
| Freezing point depression in water (°C) | 0.63655 |
| New freezing point of water (°C) | -0.63655 |
| Boiling point elevation in water (°C) | 0.175222 |
| New boiling point of water (°C) | 100.175 |
10 g of salt per litre raises the boiling point by only 0.175222 °C.
Try this example →Moles of solute per kilogram of solvent (mol/kg).
It is based on masses, which do not change with temperature, while volume does.
Kf = 1.86 K·kg/mol.
The number of particles each formula unit gives in solution: 1 for glucose, about 2 for NaCl, about 3 for CaCl₂.