Molality Calculator
Calculate molality (moles of solute per kilogram of solvent) from grams or moles, or solve for the solute or solvent mass you need. Type a formula to get the molar mass automatically, and see the freezing and boiling point of the solution.
Last updated:
Fills in the molar mass automatically
Used when the formula box is empty
1 for non-electrolytes (sugar), ≈2 for NaCl, ≈3 for CaCl₂
How to use the Molality Calculator
- Choose what to solve for: molality, amount of solute or mass of solvent.
- Enter the solute as a mass with its formula or molar mass, or as moles.
- Enter the solvent mass and pick the solvent.
- Set the van 't Hoff factor if you want freezing and boiling points (1 for sugar, about 2 for NaCl).
- Read the result and steps, then copy them or download the table.
Molality formulas
| Find | Formula | Example |
|---|---|---|
| Molality | b = n ÷ msolvent (kg) | 0.1 mol in 0.1 kg → 1.0 mol/kg |
| Moles from mass | n = mass ÷ M | 5.844 g NaCl ÷ 58.44 → 0.1 mol |
| Solute needed | n = b × msolvent | 0.5 m × 0.25 kg → 0.125 mol |
| Solvent needed | msolvent = n ÷ b | 0.2 mol ÷ 2 m → 0.1 kg |
| Freezing point depression | ΔTf = i · Kf · b | 1 m sugar in water → 1.86 °C |
| Boiling point elevation | ΔTb = i · Kb · b | 1 m NaCl (i = 2) → 1.02 °C |
Cryoscopic and ebullioscopic constants
| Solvent | Freezing point (°C) | Kf (°C·kg/mol) | Boiling point (°C) | Kb (°C·kg/mol) |
|---|---|---|---|---|
| Water | 0 | 1.86 | 100 | 0.512 |
| Benzene | 5.5 | 5.12 | 80.1 | 2.53 |
| Cyclohexane | 6.5 | 20.0 | 80.7 | 2.79 |
| Ethanol | −114.1 | 1.99 | 78.4 | 1.22 |
| Acetic acid | 16.6 | 3.90 | 118.1 | 3.07 |
| Chloroform | −63.5 | 4.68 | 61.2 | 3.63 |
Constants are typical textbook values; published figures vary slightly between sources. The ΔT formulas assume ideal, fairly dilute solutions (roughly below 1 mol/kg), so treat results at high molality as estimates.
Frequently asked questions
What is molality?
Molality (b or m) is the amount of solute in moles divided by the mass of the solvent in kilograms: b = n(solute) ÷ m(solvent). Its unit is mol/kg, often written 'm' or 'molal'. A 1 m NaCl solution has 1 mol (58.44 g) NaCl dissolved in 1 kg of water.
How do I calculate molality from grams?
First convert grams of solute to moles by dividing by the molar mass, then divide by the solvent mass in kilograms. For 5.844 g NaCl in 100 g water: 5.844 ÷ 58.44 = 0.1 mol, and 0.1 mol ÷ 0.100 kg = 1.0 mol/kg.
What is the difference between molality and molarity?
Molarity is moles per liter of solution; molality is moles per kilogram of solvent. Molality does not change with temperature because masses do not expand, which is why it is used for colligative properties such as freezing point depression.
How is freezing point depression calculated?
ΔTf = i × Kf × b, where i is the van 't Hoff factor, Kf is the solvent's cryoscopic constant (1.86 °C·kg/mol for water) and b is the molality. A 1 m sugar solution freezes at about −1.86 °C. Boiling point elevation works the same way with Kb (0.512 for water).
What is the van 't Hoff factor?
It is the number of particles a solute produces per formula unit when it dissolves. It is 1 for non-electrolytes like glucose, close to 2 for NaCl and 3 for CaCl₂. Real values are a bit lower at higher concentrations because of ion pairing.
Is molality the same as molarity for water solutions?
Only approximately, and only for dilute solutions, because 1 L of dilute solution contains about 1 kg of water. At higher concentrations they differ noticeably.
Can I use the solution mass instead of the solvent mass?
No. Molality uses the mass of the solvent only. If you know the solution mass, subtract the solute mass first.
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