About the Molality Calculator
This molality calculator works out the concentration of a solution as moles of solute per kilogram of solvent (mol/kg, written m). Enter the solute either as a mass plus its chemical formula — the molar mass is calculated for you from standard atomic weights — or directly as a number of moles, then enter the mass of solvent in grams or kilograms.
It is designed for chemistry students and lab technicians working on colligative properties, antifreeze and de-icing problems, and solution preparation. Because molality depends only on masses, it does not change with temperature, which is why it is used for freezing-point and boiling-point calculations. The calculator therefore also estimates the freezing point depression (ΔTf = i·Kf·m) and boiling point elevation (ΔTb = i·Kb·m) for water and several common solvents.
The van ’t Hoff factor i is the number of particles each formula unit produces in solution: 1 for sugars and other non-electrolytes, about 2 for NaCl and 3 for CaCl₂. The colligative estimates assume an ideal, dilute solution.
With the default inputs, the molality is 0.34223 mol/kg. Change any value above to recalculate instantly.
How to use the molality calculator
- 1Choose how you know the solute: mass and formula, mass and molar mass, or moles.
- 2Enter the solute formula or molar mass and its mass (or the moles).
- 3Enter the mass of solvent and pick g or kg.
- 4Choose the solvent and van ’t Hoff factor for freezing and boiling point changes.
- 5Read the molality in mol/kg and the colligative estimates.
Formula and method
Molality is the amount of solute in moles divided by the mass of solvent in kilograms — not the mass of the whole solution. If you give a solute mass, it is first converted to moles by dividing by the molar mass, which the calculator computes from the chemical formula using IUPAC standard atomic weights.
Freezing point depression and boiling point elevation are colligative properties: they depend on the number of dissolved particles, not their identity. Multiply the molality by the van ’t Hoff factor i and the solvent’s cryoscopic (Kf) or ebullioscopic (Kb) constant to get the temperature change. Mass percent is solute mass ÷ (solute + solvent mass) × 100.
- m
- Molality (mol/kg)
- n
- Moles of solute
- M
- Molar mass of solute (g/mol)
- i
- Van 't Hoff factor (particles per formula unit)
- Kf, Kb
- Freezing and boiling point constants of the solvent (°C·kg/mol)
Worked examples
10 g of NaCl in 500 g of water
NaCl has a molar mass of 22.99 + 35.45 = 58.44 g/mol, so 10 g is 0.1711 mol. Divided by 0.5 kg of water that is 0.342 mol/kg. With i = 2 the water freezes about 1.27 °C lower and boils about 0.35 °C higher.
0.5 mol of sucrose in 250 g of water
0.5 mol ÷ 0.25 kg = 2.0 mol/kg. Sucrose does not dissociate (i = 1), so ΔTf = 1.86 × 2 = 3.72 °C and ΔTb = 0.512 × 2 = 1.024 °C.
45 g of glucose in 1 kg of water
Glucose (C₆H₁₂O₆) has a molar mass of 180.156 g/mol, so 45 g is 0.2498 mol in 1 kg of water — a 0.250 mol/kg solution that boils about 0.128 °C higher than pure water.
Frequently asked questions
What is the formula for molality?+
Molality (m) = moles of solute ÷ kilograms of solvent. For example, 1 mol of glucose dissolved in 2 kg of water gives a 0.5 mol/kg (0.5 m) solution.
What is the difference between molality and molarity?+
Molarity is moles per litre of solution, while molality is moles per kilogram of solvent. Molality does not change with temperature because mass does not, whereas solution volume expands and contracts.
What are the units of molality?+
Molality is measured in moles per kilogram (mol/kg), often written with a lowercase italic m, as in a “0.5 m” solution. It should not be confused with M, which denotes molarity.
Why is molality used for freezing point depression?+
Colligative formulas such as ΔTf = i·Kf·m use molality because it is temperature-independent and directly counts solute particles per mass of solvent, the quantity that controls the effect.
What is the van 't Hoff factor?+
The van 't Hoff factor i is the number of particles a solute forms in solution. It is 1 for non-electrolytes like sugar, about 2 for NaCl, about 3 for CaCl₂, and a little lower than ideal in concentrated solutions.