Flexi Says: molality = ∆T / (i)(Kb for benzene) = 0.500ºC / (2.65º/m) = 0.189 molal or 0.189 moles/kg
1 kg of solvent contains 0.189 moles of unknown solute. Thus, 0.150 kg of solvent contains = 0.189 moles/kg x 0.150 kg = 0.0284 moles of unknown solute.
If the mass of the unknown solute is known, then its molar mass can be calculated using the formula:
Molar mass of solute = mass of solute/number of moles of solute