Molality Calculator

Created by Luis Hoyos
Last updated: Jul 10, 2022

Welcome to the molality calculator, a tool that will let you calculate the molality of a solution with the number of solute moles and the solvent mass. Additionally, with this calculator, you can get different molality units in your results, such as mol/kg, mol/lb, mol/oz, etc.

But don't confuse molality with molarity. If you need to calculate molarity, you can use our molarity calculator, but don't confuse them. We discuss this difference in the molarity vs. molality section, while in the other sections, we present the molality formula and applications.

💡 If you don't know how many moles of a substrate you have, use the advanced mode and calculate it using the mass and molar mass of the compound.

Molarity vs. molality

Both molarity and molality measure the concentration of a solution. However, there's one fundamental difference:

  • Molarity measures the amount of substance per unit volume of solution.
  • Molality measures the amount of substance per unit mass of the solvent. Measuring this way is sometimes convenient because of the high accuracy with which it can be determined (as opposed to molarity).

Molality formula

The definition says the molality of a solution is the amount of substance of the solute divided by the solvent mass. The equation for molality is:

b=nsolute/msolventb = n_{\text{solute}}/m_{\text{solvent}}

where:

  • bb — Molality of the solution;
  • nsoluten_\text{solute} — Number of moles of the solute; and
  • msolventm_\text{solvent} — Mass of the solvent.

As previously mentioned, we express molality as number of moles per unit mass. For example, the SI unit of molality is moles per kilogram of solvent (mol/kg)

Why do we need to calculate molality?

The calculation of molality is present in many applications:

  • Boiling point elevation: When we add a nonvolatile solute to a pure solvent (i.e., adding salt to water), the solvent boiling point increases. The higher the molality of this new solution, the greater the increase in the boiling point will be. To calculate that boiling point increase, we need to calculate the molality of the solution. Learn more about it in our boiling point elevation calculator.

  • Freezing Point Depression: Here happens something similar to what happens with boiling point elevation. When we add a nonvolatile solute to a volatile solvent, the solution's freezing point decreases compared to the pure solvent. This decrease is called the freezing point depression, and, in its equation, molality is also present. Learn more about it in our freezing point depression calculator.

  • Ionic strength: Ionic strength is the concentration of ions in a solution. To calculate it, we need to know the concentration of each ion, for which we need to know molality or molarity.

That's it for now! Hopefully, now you know the importance of molality and how to calculate it. Thanks for reading.

Luis Hoyos
Moles of solute
mol
Mass of solvent
kg
Molality
mol /
kg
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