Which is the correct unit for molality?

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

Answer: (A)

Molality expresses how many moles of solute are present for every kilogram of solvent, so the unit is mol/kg. This base is fixed by mass, not volume, which makes molality temperature-invariant (volume can expand or contract with temperature, but mass stays the same). That’s why molality is reliable when comparing concentrations at different temperatures.

Other units describe different concepts: mol/L is molarity, which uses liters of solution and changes with temperature; g/L mixes mass per volume in a way that isn’t a standard way to express concentration as a mole-based ratio; mol/solution would imply moles per total solution, which isn’t how molality is defined. So mol/kg is the correct unit.