Molality Calculator
Moles of solute per kilogram of solvent, with the mole fraction and mass percent.
Results
Molality
1.00000
mol/kg
Amount of substance
1.00000
mol
Mole fraction
0.017696
Percent by mass
5.5213
%
What this tool does
Molality counts the solvent by mass, not the solution by volume, which is the one difference from molarity and the reason it appears in freezing point and boiling point equations. In dilute water solutions the two numbers are close; in concentrated ones they separate noticeably.
Formula
b = n solute ÷ solvent mass (kg)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ms | Mass of solute | g |
mm | Molar mass | g/mol |
mv | Solvent mass | kg |
B | Molality | mol/kg |
N | Amount of substance | mol |
MF | Mole fraction | — |
PC | Percent by mass | % |
Worked example
- Mass of solute58.44 g
- Molar mass58.44 g/mol
- Solvent mass1 kg
- Molality1.00000 mol/kg
- Amount of substance1.00000 mol
- Mole fraction0.017696
- Percent by mass5.5213 %
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
Frequently asked questions
Why use molality instead of molarity?
Because molarity depends on volume, and volume changes with temperature. Heat a solution and its molarity drops slightly even though nothing was added or removed. Molality is built on mass, which does not care about temperature, so it is what the colligative property equations use. The mole fraction shown assumes water as solvent.