Molality Calculator

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 the mass of the solvent, and the tool will calculate the molality.

If you don't know how many moles of a substrate you have, you can check the Show mass of solute and molar mass checkbox to calculate it using the mass and molar mass of the compound. Are you still unsure what's the molality definition, its units, and its 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 – head over to molarity calculator.

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

Molality = n_solute / m_solvent = m_solute / (W_solute × m_solvent),

where:

• n_solute – Amount of the solute (in moles);
• m_solvent – Mass of the solvent (in kg);
• m_solute – Mass of the solute (in g); and
• W_solute – 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.

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:

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.

Let's show you an example of 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 of NaCl = 70.128 g / (58.44 g/mol) = 1.2 mol.

4. Plug the number of moles 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 advanced mode to enter also the molar mass and solute mass).