Molar mass is the mass (in atomic mass units) of one mole of a substance. The molecular weight of a molecule is the total number of protons and neutrons in all the atoms in that molecule. The freezing point of a solution is lower than that of the pure solvent, and the freezing-point depression (DeltaT) is directly proportional to the amount concentration for dilute solutions. When the amount concentration is expressed as a molality, the proportionality constant is known as the cryoscopic constant (Kf) and is characteristic for each solvent. If w represents the mass fraction of the solute in solution, and assuming no dissociation of the solute, the molar mass is given by the formula.