Molality Calculator

The molality formula

Molality is defined as:

m = moles of solute (n) / mass of solvent (kg)

The unit is mol/kg, often written as m (italic, lowercase) and sometimes called molal. Note carefully: the denominator is the mass of the solvent alone, the water, ethanol or other liquid you dissolve into, not the mass of the whole solution. This is the single most common mistake when computing molality.

Worked example

Suppose you dissolve 0.5 mol of glucose in 2 kg of water:

• Moles of solute: 0.5 mol
• Mass of solvent: 2 kg
• Molality: 0.5 ÷ 2 = 0.25 mol/kg

So the solution is 0.25 molal. If you instead had 1 mol of solute in 0.5 kg of solvent, the molality would be 1 ÷ 0.5 = 2 mol/kg.

Reference table

Molality vs. molarity

• Molality (m): moles per kilogram of solvent. Mass-based, so it is temperature-independent, heating or cooling does not change it.
• Molarity (M): moles per liter of solution. Volume-based, so it changes with temperature as the liquid expands or contracts.

For dilute aqueous solutions near room temperature the two values are numerically close because 1 L of water weighs about 1 kg, but they diverge for concentrated solutions, non-aqueous solvents, or precise work.

Common pitfalls

• Using the solution mass instead of the solvent mass. Always weigh the solvent on its own, or subtract the solute mass from the total.
• Mixing grams and kilograms. The formula needs kilograms; divide grams by 1000 first.
• Confusing m and M. Lowercase m is molality (per kg solvent); uppercase M is molarity (per L solution).
• Forgetting to convert mass to moles. If you start from grams of solute, divide by the molar mass (g/mol) before entering the value here.