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Calculator Answers

Molality Calculator

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

A molality calculator is a chemistry measurement tool that computes the molal concentration of a solute in a solvent. Molality is defined as the number of moles of solute divided by the mass of the solvent in kilograms, distinct from molarity which depends on solution volume. The calculator processes input values of solute mass, molar mass, and solvent mass, executing the necessary conversions to produce the final molality value. Chemists and chemistry students use this tool to study colligative properties like boiling point elevation.

Pick a mode, enter the known values, and the calculator returns molality, moles, or kg solvent using m = moles solute / kg solvent. Optionally enter solute as grams plus molar mass.

Quick Answer

Calculate solution molality, solute moles, or solvent mass. Enter the solute and solvent amounts to find the concentration of your solution.

Solve for

Solute input

mol

Unit

Molality vs molarity

Molality (m) uses moles of solute per kilogram of solvent, a mass-based unit that does not change with temperature. Molarity (M) uses moles per liter of solution, which is volume-based and shifts with temperature as the solution expands or contracts. They are not interchangeable.

Result

Molality

0.5 mol/kg (m)

Formulam = moles solute / kg solvent = 0.5 / 1 = 0.5 mol/kg

Molality is preferred for colligative properties like boiling-point elevation and freezing-point depression, which depend on the solvent mass rather than the solution volume.

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Examples

0.5 mol solute in 1 kg solvent

= 0.5 m

29.22 g NaCl in 1 kg water (M=58.44)

≈ 0.5 m

Target 0.25 m with 0.1 mol solute

needs 0.4 kg solvent

2 m molality x 0.5 kg solvent

= 1 mol solute

How it works

Molality is a single-formula relation between moles of solute and mass of solvent. Solve for any of the three given the other two.

Molality · m = moles solute / kg solvent

Solute from grams · n = mass (g) / molar mass (g/mol)

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Frequently asked questions

Molality (m) is concentration measured in moles of solute per kilogram of solvent (mol/kg). A 1 m solution has 1 mole of solute dissolved in 1 kilogram of solvent (not solution).

Molarity (M) is moles per liter of solution and is volume-based. Molality is moles per kilogram of solvent and is mass-based. Molality does not change with temperature, while molarity does because the solution volume expands or contracts.

For colligative properties (boiling-point elevation, freezing-point depression, vapor-pressure lowering, osmotic considerations in some contexts), use molality. For most other intro chemistry problems, molarity is more common.

Molality uses only the solvent's mass. That makes the unit temperature-independent and decouples it from solute-driven volume changes. Molarity, by contrast, depends on the total solution volume.

Yes. Switch the solute input to 'Grams + molar mass'. The calculator converts grams to moles using n = m / M, then plugs into the molality formula.

Molality in mol/kg, also written 'm' (lowercase italic in formal notation). 1 m solution is read as 'one molal' to avoid confusion with 1 M (one molar).