One mole of dissolved substance in 0.5 kg of solvent has a molality of 2 mol/kg. Use the mass of solvent, not the mass of the entire solution, in the denominator.
Why this works
Molality = moles of solute / kilograms of solvent. Molarity = moles of solute / litres of solution. If you start from solute mass, first divide by its molar mass to obtain moles. Keep solvent and solution quantities clearly labelled.
When this answer changes
Molarity uses solution volume, which can vary with temperature. Molality uses solvent mass. You cannot generally convert one to the other using a factor of 1,000: conversion needs additional information such as solution density and composition.
Try your own values
Use the molality calculator to test your inputs. Save two scenarios to compare results, or copy an input link to revisit your calculation.
