Molality Calculator
Find molality from moles of solute and kg of solvent.
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How it works
Molality expresses the concentration of a solution as moles of solute per kilogram of solvent, and this calculator finds it directly from those two quantities. You enter the moles of dissolved solute and the mass of the solvent in kilograms, and it returns the molality in moles per kilogram. Molality is distinct from the more familiar molarity, which is moles per litre of solution: molality is based on the mass of solvent alone rather than the total volume. This difference matters because mass does not change with temperature, so molality stays constant even as a solution is heated or cooled, whereas molarity shifts because liquids expand and contract. That temperature independence makes molality the concentration unit of choice for colligative property calculations such as boiling point elevation and freezing point depression, which are central to understanding antifreeze, salting icy roads, and osmosis. Chemistry students and researchers use molality whenever temperature stability matters or when working with those colligative effects. To calculate it, be sure to measure the solvent mass, not the solution mass, and convert the solvent to kilograms. Enter the moles of solute and the solvent mass to get the molality.
molality = moles of solute / kilograms of solvent
Molality is the number of moles of solute divided by the mass of solvent in kilograms, giving concentration in moles per kilogram. Unlike molarity, which uses the volume of the whole solution, molality uses only the solvent mass, so it does not change with temperature because mass, unlike volume, does not expand or contract when heated or cooled. This makes molality the preferred concentration unit for temperature-dependent calculations.
Source:Chemistry (solution concentration)
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Worked example
How to use this calculator
- 1
Enter your values
Enter moles of solute and mass of solvent.
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Adjust the options
Fine-tune any options to match your situation.
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Read your result
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Frequently asked questions
What is molality?
Molality is a measure of concentration defined as the number of moles of solute per kilogram of solvent, with units of moles per kilogram, sometimes written molal. It differs from molarity by using the mass of the solvent rather than the volume of the solution. Because it depends on mass, molality does not change with temperature, unlike volume-based concentrations.
How is molality different from molarity?
Molality is moles of solute per kilogram of solvent, while molarity is moles of solute per litre of solution. The key practical difference is that molality uses the solvent mass and is temperature-independent, whereas molarity uses solution volume and changes as temperature alters the volume. For dilute aqueous solutions near room temperature, the two values are often numerically close.
Why does molality not change with temperature?
Molality is based on mass, and mass does not change when a substance is heated or cooled. Volume, by contrast, expands as temperature rises and contracts as it falls, so any concentration based on volume, like molarity, shifts with temperature. Because molality avoids volume entirely, it stays fixed, which is why it is used in temperature-sensitive calculations.
When should I use molality instead of molarity?
Use molality when temperature will change or when calculating colligative properties such as boiling point elevation, freezing point depression, and osmotic effects, which depend on the amount of solute relative to solvent mass. Molarity is more convenient for everyday solution preparation where you measure by volume. The choice depends on whether temperature independence matters for your application.
Do I use solvent or solution mass?
Molality uses the mass of the solvent alone, not the total mass of the solution. This is a common mistake: you must weigh or calculate just the solvent, such as the water, and not include the dissolved solute in that mass. Convert the solvent mass to kilograms before dividing the moles of solute by it.