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Molality Calculator

Find molality (moles of dissolved substance per kilogram of liquid), or how much of either you need for a target molality.

Level: Grades 11–12

Printable worksheet

Molality Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Solve for
Measure the solute in

Grams per mole of the dissolved substance; sodium chloride is 58.44 g/mol.

The solvent only (for example the water), not the whole solution.

Result

Molality (b)

0.4 mol/kg

Molality (b) is 0.4 mol/kg.

Moles of solute
0.2 mol
Mass of solvent
0.5 kg

See the step-by-step solution ↓

Step-by-step solution

  1. Turn grams into moles: divide by the molar mass, n = mass ÷ molar mass

    n=11.688 g58.44 g/mol=0.2 mol

  2. Molality uses kilograms, so divide the grams by 1,000

    500 g1,000=0.5 kg

  3. Molality is moles per kilogram. So divide the moles by the kilograms: b = n ÷ m

    b=0.2 mol0.5 kg=0.4 mol/kg

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how concentrated a solution is, measured against the weight of the liquid it's dissolved in. That measure is called molality. It's the moles of dissolved substance (the solute) per kilogram of the liquid doing the dissolving (the solvent). A mole is a fixed count of particles, used to measure amounts in chemistry. Molarity, a related measure, divides by the volume of the whole solution instead. Molality uses a mass, and masses don't change when a solution warms up or cools down. That makes molality the natural unit for boiling-point rise, freezing-point drop and other work that depends on temperature.

This calculator finds the molality of a solution. It can also find the amount of solute needed to reach a target molality, or the mass of solvent to use. You can give the solute in moles, or in grams along with its molar mass (grams per mole). Give the solvent in grams or kilograms.

Where it's used

Why this math matters outside the classroom:

  • Freezing points. Chemists use molality for freezing-point and boiling-point changes because, unlike volume, it doesn't shift with temperature.
  • Road salt. Road crews rely on salt lowering water's freezing point by an amount that grows with the molality of dissolved ions.
  • Lab preparation. Dissolving 58.44 g of sodium chloride in 2 kg of water gives a molality of 1 mol ÷ 2 kg = 0.5 mol/kg.
  • Ice cream makers. Ice cream makers add salt to the ice around the churn, lowering its melting point so the mix freezes faster.

How to use it

  1. Choose what to solve for: molality, the solute amount or the solvent mass.
  2. Choose whether you measure the solute in grams (you'll also need its molar mass) or in moles.
  3. Enter the values you know. For the solvent, pick grams or kilograms beside the field.
  4. Read the answer. The working shows any grams-to-moles or grams-to-kilograms conversion.

What the inputs mean

Molality (b)
The concentration you want, in moles of solute per kilogram of solvent (mol/kg, sometimes written m).
Mass of solute
The mass of the substance being dissolved, in grams.
Molar mass of solute
Grams per mole of the solute, from its chemical formula. Sodium chloride (table salt) is 58.44 g/mol; glucose is 180.16 g/mol.
Mass of solvent
The mass of the solvent alone, before you add the solute. For water, 1 mL weighs very close to 1 g at room temperature.

Formula and method

b = n ÷ m
n = mass of solute ÷ molar mass
n = b × m        m = n ÷ b
m in kg = m in g ÷ 1,000

Here b is the molality, n is the amount of solute in moles and m is the mass of solvent in kilograms. Every mode uses this same formula, just rearranged.

If you enter the solute in grams, the calculator first divides by the molar mass to get moles. When it solves for the solute, it multiplies the moles back by the molar mass to give grams.

You divide by the solvent, not the whole solution. Adding 58.44 g of salt to 1 kg of water gives exactly 1 mol/kg, even though the total mass is 1.058 kg.

Worked example

You dissolve 11.688 g of sodium chloride (58.44 g/mol) in 500 g of water. What is the molality?

  1. Moles of NaCl: 11.688 ÷ 58.44 = 0.2 mol.
  2. Solvent mass in kilograms: 500 ÷ 1,000 = 0.5 kg.
  3. Molality: 0.2 ÷ 0.5 = 0.4 mol/kg.

Result: The solution is 0.4 mol/kg (0.4 molal) sodium chloride.

Understanding the result

A 1 mol/kg solution has one mole of solute for every kilogram of solvent. In weak water solutions, molality and molarity are nearly equal. That's because a liter of solution holds close to a kilogram of water. They drift apart as solutions get stronger, or when the solvent isn't water.

Some properties, like how far the freezing point drops, depend on the number of dissolved particles. Chemists call these colligative properties. They grow with molality times the number of particles each formula unit splits into. A 0.4 mol/kg NaCl solution behaves roughly like 0.8 mol/kg of particles, since each unit splits into Na⁺ and Cl⁻.

Assumptions and limitations

  • The solvent mass is the mass of pure solvent, not counting the solute.
  • The molar mass you enter matches the exact form of the solute you used. That includes any water built into its crystals (water of crystallization).
  • The calculator doesn't convert between molality and molarity, because that needs the solution's density.
  • It reports the molality of the whole formula unit, not of the separate charged particles (ions) it forms in solution.
  • It handles one solute at a time. For mixtures, work out each solute separately.

Frequently asked questions

What's the difference between molality and molarity?

Molality is moles per kilogram of solvent. Molarity is moles per liter of the whole solution. Molality doesn't change with temperature because mass doesn't. Volume, on the other hand, expands and shrinks.

Why does molality use the solvent instead of the solution?

Using only the solvent makes a solution easy to prepare. You weigh the solvent and the solute separately, then mix them. It's also the amount that appears directly in the equations for boiling-point and freezing-point changes.

What does 'm' mean after a number?

A lowercase m, as in 0.4 m, is an older shorthand for molal, meaning mol/kg. It's easy to confuse with meters or with molar (M), so mol/kg is clearer.

How this calculator is checked

Method: Definition of molality b = n_solute / m_solvent (IUPAC); n = mass / molar mass. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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