About this tool
Find the molality of a solution — moles of solute per kilogram of solvent — from masses, or convert between molality and molarity using the solution density. All math runs in your browser with nothing uploaded.
From masses the molality is b = (mass of solute / molar mass) / (mass of solvent / 1000), where masses are in grams. To interconvert with molarity, the tool uses b = M / (ρ − M × Mr / 1000) to go from molarity to molality, and M = b × ρ / (1 + b × Mr / 1000) for the reverse, where ρ is the solution density in g/mL (equal to kg/L) and Mr is the molar mass in g/mol.
Molality is preferred for colligative properties such as freezing-point depression and boiling-point elevation because it depends on mass and does not change with temperature, unlike molarity which uses volume. Choose the mode, enter your values, and the molality, moles of solute and molarity are shown.