Molality Calculator

m = moles solute / kg solvent. Used in colligative-property calculations (boiling-point elevation, freezing-point depression).

Inputs

Result

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How to use this calculator

  • Enter moles of solute and kilograms of solvent.
  • Read molality.

About this calculator

Molality is moles of solute per kilogram of solvent (note: solvent only, not solution). Unlike molarity, molality doesn't change with temperature because mass doesn't change with temperature. Used in colligative properties: ΔT_b = K_b × m, ΔT_f = K_f × m.

Frequently asked

Molality vs molarity?+
Molality uses solvent mass, molarity uses solution volume. Molality is constant with temperature; molarity isn't.
When use molality?+
Boiling-point elevation, freezing-point depression, osmotic-pressure calculations — anywhere temperature varies.
Are molality and molarity ever equal?+
For dilute aqueous solutions at 4°C they're very close (water density ≈ 1.00 g/mL). For concentrated or non-aqueous solutions they differ significantly.
How does adding solute affect kg?+
kg refers to solvent only — adding solute doesn't change the solvent mass. Total solution mass = solvent mass + solute mass.
Why "m" lowercase?+
Capital M is reserved for molarity. Lowercase m for molality. Don't confuse them.

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