Free · Stoichiometry calculators 🧪

Solute Moles, Solvent Mass & Molality

Calculate molality from solute amount and solvent mass, with an optional solute-mass and molar-mass calculation.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

Dissolving 0.5 mol of solute in 2 kg of solvent gives molality 0.25 mol/kg. The denominator is solvent mass alone.

Molality = moles of solute / solvent mass; moles = solute mass / molar mass

Worked example

Enter these known values and leave the other values blank.

Amount of solute
0.5 mol
Solvent mass only
2 kg
Molality
0.25 mol / kg

Assumptions and limitations

  • All quantities are positive. Use solvent mass, excluding the mass of solute.
  • The reciprocal mass selector reports moles per selected solvent-mass unit. The usual molality unit is mol/kg; it is different from molarity in mol/L.
  • For the mass-based option, provide molar mass in g/mol for the specified solute entity. This calculator does not infer dissociation or count dissolved particles automatically.

Common questions

Why is solution mass the wrong denominator?

Molality is defined per mass of solvent. Including solute mass would calculate a different ratio.

Does thermal expansion change molality?

For a closed mixture with unchanged component amounts, thermal volume expansion does not change the mass-based molality.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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