Molality of a solution is defined as the number of moles of solute per:

1. Molality (m) = moles of solute / mass of solvent in kg. 2. Molarity uses volume of solution in litres, so option A describes molarity, not molality. 3. The defining mass is that of the solvent (not the solution), so molality is moles of solute per kilogram of solvent. _Source: syllabus Chemistry Ch 1 "Solutions", p.4_