Question

PLEASE SHOW ALL WORK

Determine the molar mass of a solute if enough of it is added to 20 grams of benzene so that the total mass of the mixture is 22.5 grams and the freezing point is 3.5 degrees celcius. Assume that the solute dissolves without dioociating into smaller pieces.

Answer 1

mass of solvent benzene = 20 g

mass of solute = 22.5 - 20 = 2.5 g

dTf = To -Tf

      = 5.5 - 3.5

       = 2.0 oC

Kf = 5.12 oC / m

delta Tf = Kf x molality

2.0   = 5.12 x molality

molality = 0.391 m

moles = molality x mass of solvent (kg)

          = 0.391 x 20 x 10^-3

         = 7.81 x 10^-3

mass / molar mass = 7.81 x 10^-3

2.5 / molar mass = 7.81 x 10^-3

molar mass = 320 g / mol

molar mass of solute = 320 g / mol