Molality Calculator

Calculate the molality of a solution by entering the amount of solute and mass of solvent below.

Molality:

  mol/kg


How to Calculate Molality

Molality is a measure of the concentration of a compound in a solution measured in the amount of substance in moles by the mass of solute in kilograms. Molality is often expressed in moles per kilogram, or mol/kg, and is denoted using the letter m.

First used in 1923, molality is different from molarity in that it’s the ratio of the amount of substance of a solute to the mass of solvent, while molarity is the amount of substance of the solute to the volume of solvent.[1]

Molality Formula

Since it’s a ratio, you can calculate molality using the following formula:

molality (m) = moles of solutekilograms of solvent

Thus, the molality m is equal to the amount of substance of solute in moles divided by the mass of solvent in kilograms.

For example, let’s calculate the molality of a solution with 3 moles of solute and 2 kilograms of solvent.

molality (m) = 3 moles solute2 kg solvent
molality (m) = 32 = 1.5 mol/kg

In this example, the molality of the solution is equal to 1.5 mol/kg.

References

  1. Lewis, G. N., Randall, M., Thermodynamics and the Free Energy of Chemical Substances, McGraw-Hill Book Company, Inc., 1923, New York and London, https://books.google.com/books?id=m81AAAAAIAAJ&ots=K6ziAMlj7Y&lr&pg=PR7