# What is the molality of my solution?

Computes molality, moles of solute per kilogram of solvent, from the solute mass and molar mass or its moles, or works out the solute mass needed for a target molality.

- Page: https://www.acalculator.org/chemistry/molality-calculator
- JSON spec: https://www.acalculator.org/chemistry/molality-calculator.json
- Version: 6f354d6c03ed

## Default answer

Example with the default inputs (Find Molality from mass, Solute mass 2,220 g, Molar mass, g/mol 62.07, Solvent mass 2,000 g): The answer is 17.883 mol/kg.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| find | Find | The molality from the solute mass or its moles, or the solute mass needed for a molality. |
| ms | Solute mass | The mass of the dissolved substance. |
| n | Solute, mol | The amount of the dissolved substance in moles. |
| b | Molality, mol/kg | The molality you want, in moles of solute per kilogram of solvent. |
| M | Molar mass, g/mol | The molar mass of the solute in grams per mole (58.44 for sodium chloride). |
| mw | Solvent mass | The mass of the solvent alone (for example the water), not of the whole solution. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| result | Answer | The molality or the solute mass, to 6 significant figures. |
| molality | Molality (mol/kg) | Moles of solute per kilogram of solvent. |
| moles | Solute (mol) | The amount of solute in moles. |
| grams | Solute (g) | The solute mass in grams (when the molar mass is known). |
| mmolal | Molality (mmol/kg) | The molality in millimoles per kilogram. |

## Method

molality = moles of solute ÷ kilograms of solvent; moles = grams ÷ molar mass. For a target molality: moles = molality × kg of solvent; grams = moles × molar mass.

## Assumptions

- The solvent mass is the solvent alone, not the solution. Molality does not change with temperature, because it uses masses, not volumes.

## Worked examples

1. find = mass, ms = 2.22, M = 62.07, mw = 2 gives b = 17.883035, result = 17.883 mol/kg. Source: OpenStax, Chemistry 2e, §11.4 Colligative Properties (molality m = mol solute ÷ kg solvent), https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties, Example 11.3 Calculating Mole Fraction and Molality: 17.9 m.
2. find = solute, b = 0.5, M = 58.44, mw = 0.25 gives grams = 7.305, n = 0.125, result = 7.305 g of solute (0.125 mol). Source: OpenStax, Chemistry 2e, §11.4 Colligative Properties (molality m = mol solute ÷ kg solvent), https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties.
3. find = mol, n = 0.3, mw = 0.6 gives b = 0.5, mmolal = 500. Source: OpenStax, Chemistry 2e, §11.4 Colligative Properties (molality m = mol solute ÷ kg solvent), https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties.

## FAQ

### What is molality?

Molality (m) is the moles of solute per kilogram of solvent: m = mol solute ÷ kg solvent. A 1 m solution has one mole of solute for each kilogram of solvent.

### How is molality different from molarity?

Molarity divides by the liters of solution; molality divides by the kilograms of solvent alone. Volumes change with temperature and masses do not, so molality stays the same when a solution warms or cools.

### How do I calculate molality from grams?

Divide the solute grams by its molar mass to get moles, then divide by the solvent mass in kilograms. 2,220 g of ethylene glycol (62.07 g/mol) in 2.00 kg of water is 35.77 mol ÷ 2.00 kg = 17.9 m.

### How much solute do I need for a given molality?

Multiply the molality by the kilograms of solvent to get moles, then by the molar mass. A 0.500 m salt solution in 250 g of water needs 0.125 mol, or 7.305 g of NaCl.

### Should I use the mass of the solution or of the solvent?

The solvent. If you know only the solution mass, subtract the solute mass first.

### Where is molality used?

In colligative properties: the boiling point rise and freezing point drop of a solution are proportional to its molality, as in ΔT_f = K_f × m.

## Sources

- OpenStax, Chemistry 2e, section 11.4 Colligative Properties: molality m = mol solute ÷ kg solvent; Example 11.3 (ethylene glycol, 17.9 m). CC BY 4.0, retrieved 2026-10-03. https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties
- NIST Special Publication 811, Appendix B.8: 1 lb = 0.45359237 kg; 1 oz = 0.028349523125 kg. Retrieved 2026-10-03. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8