Molality Calculator
Molality from moles of solute and kilograms of solvent, from a mass or from moles directly.
What is Molality?
Molality is moles of solute per kilogram of solvent — the solvent you weighed out, not the solution volume you made up. That is the whole difference from molarity: a volumetric flask measures litres of solution, a balance measures kilograms of solvent, and the two concentrations drift apart as the solution is heated or concentrated. Molality does not drift with temperature, which is why colligative-property work is done in it.
How it works
m = n solute ÷ kg solvent, with the solvent unit selectable between grams and kilograms so a '500 g of water' problem needs no manual conversion. Given a mass of solute, the molar mass converts it to moles first, and that step is shown in the working rather than hidden. The result page carries a one-line molarity-vs-molality clarifier that links the molarity page, because the two are conflated constantly and the pages meet in the middle.
- Enter what you know. Every field recalculates as you type — there is no submit button. Leave the quantity you want blank and the page solves for it.
- Read the answer and the working. The result appears above the fold; underneath it is the formula with your numbers substituted, so the arithmetic can be checked rather than trusted.
- Check the units and the size. Units are shown on every value and selectable per field, so a result in kJ, kPa or km/h is converted rather than assumed. Sanity-check the magnitude against a known landmark such as 1 g/cm³ for water.
Examples
A textbook problem with a known answer
Every tool here is checked against standard example problems — the molar mass of C6H12O6 is 180.16 g/mol, and free fall from 20 m takes about 2.02 s at g = 9.8 m/s².
A result you want to reason backwards from
Leave the quantity you already know blank instead: given the final answer, most tools recover the starting value — the mass needed for a target concentration, or the height a fall must have had.
Common mistakes
Forgetting that a formula is only as good as its subscripts
H2O and H2O2 are different compounds. The formula field shows its parse — element by element — so a mis-typed subscript is visible before it reaches the arithmetic.
Mixing units in the middle of a calculation
The most common source of a wrong answer is a value in grams next to a formula expecting kilograms, or Celsius next to a gas constant expecting kelvin. This category converts on the way in and labels what it used.
Frequently asked questions
Molarity vs. molality — which denominator is which?
Molarity divides by litres of solution (solute plus solvent together); molality divides by kilograms of solvent alone. 0.5 mol in 500 mL of solution is 1.0 M; the same 0.5 mol dissolved in 500 g of water is 1.0 m only if the solution volume stays 500 mL, which it almost never does exactly.
Why does molality not change with temperature?
Because both of its quantities are masses. Liquids expand as they warm, so a molarity drifts downward in a heated solution; kilograms do not expand, so a molality stays put. That is why freezing-point and boiling-point calculations are written in molality.
Does the solvent mass include the solute?
No — it is the mass of the solvent by itself, before anything is dissolved in it. Weigh the water, not the beaker with the salt already in it.