What are the units of molality?

• A. mol/L
• B. mol/kg
• C. g/L
• D. dimensionless

Answer: (B)

Molality measures how many moles of solute are present per kilogram of solvent. Its unit is mol/kg because it uses the mass of the solvent, not the total solution, as the reference. This is different from molarity, which is moles per liter of solution, and from a mass concentration like g/L. Since mass is essentially unaffected by typical temperature changes, molality stays constant with temperature, making it especially useful in colligative-property calculations. For example, dissolving 1 mole of solute in 1 kilogram of solvent gives 1 molal (1 mol/kg).