Molality Calculator

Created by Hanna Pamuła, PhD
Reviewed by Bogna Szyk and Jack Bowater
Last updated: Nov 27, 2022

With this molality calculator you can quickly calculate the molality - one way of measuring the concentration of a solute in a solution (not to be confused with molarity). Simply type the number of moles of your solute substance and mass of the solvent and the tool will calculate the molality. If you don't know how many moles of a substrate you have, use the advanced mode to calculate it with the use of mass and molar mass of the compound. Are you still unsure what's the molality definition, its units and formula - or simply how to calculate molality step by step? Keep reading and you'll find the answer!

If you were looking for a tool to calculate molarity - go to molarity calculator.

Molality definition and molality formula

Molality, also called molal concentration, is defined as the amount of substance of solute, divided by the mass of the solvent msolvent:

Molality = nsolute / msolvent = msolute / (Wsolute × msolvent)

where

  • nsolute is amount of the solute (in moles)
  • msolvent is a mass of the solvent (in kg)
  • msolute is a mass of the solute (in g)
  • Wsolute is a molar mass of the solute (in g/mol).

The molality unit from the SI system is mol/kg; sometimes, the name molal is used (though it's considered obsolete).

Molality plays a significant role in calculating the ionic strength of a solution. Learn more in the ionic strength calculator.

What is the difference between molarity and molality?

Molarity and molality are similar concepts - both are measures of the concentration of a solution. However, there is one main difference between those terms: molarity is expressed as the amount of substance per unit volume of solution, whereas molality defines the concentration as the amount of substance per unit mass of the solvent.

We've put all the most important differences between those two terms into a short molarity vs. molality table:

Molarity

Molality

Definition

Amount of substance (in moles) divided by the volume (in litres) of the solution.

Amount of substance (in moles) divided by the mass (in kg) of the solvent.

Symbol

M

m or b

Unit

mol/L

mol/kg

Temperature and pressure

dependent

independent

Usage

more popular, practical to use in the lab, faster and easier

accurate but rarely used

How do I convert molality to molarity?

To change between molarity and molality, you can use the following relation:

M = m × d / (1 + m × W)

where

  • M - molarity;
  • m - molality;
  • W - the molar mass of the solute; and
  • d - mass density of the solution.

How to calculate molality with this molality calculator

Let's show on the example how to calculate molality:

  1. Choose your substance. Let's assume it's table salt - sodium chloride (NaCl).
  2. Calculate its molar mass. For sodium chloride it's equal to 58.44 g/mol (as Na = 22.99 g/mol and Cl = 35.45 g/mol).
  3. Convert grams to moles. Use the formula moles = mass of solute / molar mass. Assume we want to dissolve 70.128 grams of salt in 1.5 kg of water. so moles NaCl = 70.128 g / (58.44 g/mol) = 1.2 mol.
  4. Plug moles value and the mass of the solvent into the molality formula. Divide 1.2 mol by 1.5 kg, and you'll find out that the molality of the NaCl solution is 0.8 molal (in standard molality units: 0.8 mol/kg).
  5. Or save yourself some time and use our molality calculator (choose an advance mode to enter also the molar mass and solute mass).

FAQ

How do I calculate molality?

To compute the molality of a solute, you need to:

  1. Divide the amount of the solute (in moles) by the mass of the solvent (in g).
  2. Alternatively, if the amount of the solute is unknown, multiply the mass of the solvent (in g) and its molar mass (in g/mol).
  3. Then divide the mass of the solute (in g) by the result from Step 2.
  4. That's it! You've just determined the molality.
  5. Remember to include the units in your answer: it's mol/kg or molal.

What is the molality if there are 3 moles in 0.5 kg of solvent?

The answer is 6 mol/kg. To see how this result is derived, recall that the formula for molality reads molality = moles of solute / mass of solvent in kg. Plugging in moles of solute = 3 and mass of solvent = 0.5 kg, we obtain molality = 3 / 0.5 = 6 mol/kg, as claimed.

How do I convert molarity to molality?

The formula that allows us to convert molarity to molality is the following:

m = M/( d - M × W)

where:

  • M - molarity;
  • m - molality;
  • W - the molar mass of the solute; and
  • d - mass density of the solution.
Hanna Pamuła, PhD
Moles of solute
mol
Mass of solvent
kg
Molality
mol /
kg
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