Molality Calculator
Molality in mol/kg from the mass or moles of a solute and the mass of solvent, with the matching molarity from the solution’s density, or molality back from a molarity.
Result…
How it's calculated
Molality m = moles of solute ÷ kilograms of solvent Moles = mass ÷ molar mass Molarity M = 1,000 × ρ × m ÷ (1,000 + m × molar mass) (ρ in g/mL) Back: m = 1,000 × M ÷ (1,000 × ρ − M × molar mass)
Molality (symbol m, unit mol/kg) is the amount of solute divided by the mass of the solvent alone. Because it uses masses, it stays the same as the temperature changes, which is why it appears in the formulas for boiling point elevation and freezing point depression. Molarity divides by the volume of the whole solution instead. To convert, the calculator takes a kilogram of solvent, adds the solute’s mass (moles × molar mass), and turns that total mass into a volume with the solution’s density. Going back, it takes one litre of solution, subtracts the solute’s mass from the litre’s mass to get the solvent. Mass units are converted exactly (1 lb = 453.59237 g).
Your numbers, step by step
About the Molality calculator
This molality calculator finds the concentration of a solution in moles of solute per kilogram of solvent. Enter the mass of the solute and its molar mass, or the moles directly, together with the mass of the solvent on its own, and it gives the molality in mol/kg and mmol/kg plus the mass percent. Add the density of the finished solution and it also works out the solution volume and the molarity in mol/L. You can go the other way too: start from a molarity, the molar mass and the density, and it returns the molality. Chemistry students use it for freezing point and boiling point problems, where molality is the unit the formulas expect, and lab staff use it to switch between the two concentration scales. Every conversion is written out in the steps with your numbers.
How to use it
- Under “Work from”, choose “Mass of solute”, “Moles of solute” or “A molarity”.
- Enter the “Mass of the solute” with its unit and the “Molar mass of the solute” in g/mol, or the “Moles of the solute”.
- Enter the “Mass of the solvent”: the water or other solvent alone, not the finished solution.
- To get molarity as well, set “Convert with the density” to “Yes” and enter the “Density of the solution” with its unit.
- Read the molality, then the molarity, solution volume and mass percent below it.
Worked example
18 g of glucose (molar mass 180.156 g/mol) is dissolved in 500 g of water. Moles = 18 ÷ 180.156 = 0.09991 mol, and 500 g is 0.5 kg, so molality = 0.09991 ÷ 0.5 = 0.1998 mol/kg. The solution weighs 518 g and is 3.47% glucose by mass. With a measured density of 1.012 g/mL its volume is 518 ÷ 1.012 = 511.9 mL, so the molarity is 0.09991 ÷ 0.5119 = 0.1952 mol/L.
Updated
Questions people ask
How do I calculate molality?
Divide the moles of solute by the mass of the solvent in kilograms. If you have grams of solute, first divide by the molar mass to get moles.
What is the difference between molality and molarity?
Molality is moles per kilogram of solvent; molarity is moles per litre of the whole solution. Molality does not change with temperature, while molarity does because volume expands. The molarity calculator works out masses to weigh for a target molarity.
How do I convert molarity to molality?
Take one litre of solution: its mass is the density × 1,000 mL. Subtract the solute’s mass (molarity × molar mass) to get the solvent’s mass, then divide the moles by that mass in kilograms.
Why do freezing point formulas use molality?
Because it is based on masses, so it stays fixed as the solution is heated or cooled. The colligative properties calculator uses it to find boiling and freezing point changes.
What mistake do people make most with molality?
Using the mass of the whole solution instead of the solvent alone. The kilograms in the formula are the solvent only, so subtract the solute’s mass if you weighed the finished solution.
Is the molarity it gives exact?
Only as good as the density you enter. Density changes with temperature and concentration, so use a value measured or published for your solution at its temperature.