Molality Calculator

A Concentration Unit That Doesn't Care About Temperature

Molarity has a hidden flaw: it's defined per liter of solution, and liquids expand and contract with temperature, so a solution's molarity technically drifts as the thermometer moves. Molality sidesteps that entirely by measuring against the mass of solvent instead of the volume of solution — mass doesn't change with temperature, which makes molality the preferred unit for precise physical chemistry work like freezing-point depression and boiling-point elevation.

The Formula

Note the denominator is the mass of solvent alone, not the total mass or volume of the resulting solution — a subtle but important distinction from molarity.

Where This Calculation Matters

• Colligative property calculations — freezing-point depression and boiling-point elevation formulas are both defined in terms of molality specifically.
• Precise physical chemistry measurements — molality's independence from temperature and pressure makes it the standard unit in thermodynamic and calorimetric work.
• Antifreeze and de-icing formulations — calculating how much freezing-point depression a given molality of solute provides.
• Osmotic pressure and vapor pressure studies — molality is used where a temperature-stable concentration unit is required.

Worked Examples

For dilute aqueous solutions near room temperature, molality and molarity are numerically close because a liter of water weighs almost exactly a kilogram — the two diverge more at higher concentrations or temperatures.

How to Use This Calculator

1. Enter the Moles of Solute (mol) dissolved.
2. Enter the Mass of Solvent (kg) — the solvent only, not the total solution mass.
3. Select Calculate to get the molality in mol/kg.

Related Calculations

Need moles from a measured mass of solute first? Use the Moles Calculator. For the more commonly used volume-based concentration unit, see the Molarity Calculator.