Question

A 2.91M ferric chloride solution freezes at -4.74 oC. (K_f,H2O=1.86 oC/m, K_b,H2O =0.512 oC/m, i_FeCl3=3.4)

a) Calculate the molality of ferric chloride in the solution.

b) Determine the boiling point of the solution.

c) Calculate the osmotic pressure of this solution at 306K.

Answer 1

a. Δt = - i.Kf.m

  m = Δt / (i.Kb) = 4.74^oC/(3.4x1.86 ^oC/m) = 0.75 m

b. Δt = i.Kb.m

         = 3.4 x o.512 ^oC/m x 0.75 m = 1.3 ^oC

    BP = 100 ^oC + 1.3 ^oC = 101.3 ^oC

c.

Without a value for the density, assume it to be close to 1.(or take it from your book, if any) . In this case molality (mol/1000g solvent) = molarity (mol/L)

Π= i (n/V) x RT

    = 3.4 mol^-1 x (0.74 mol/1 L) x 0.0821 L.atm.K^-1 x 306 K = 63 atm