Question

ure cyclohexane (C6H12) boils at 81.0°C. What is the boiling point of a solution in which 40.0 g of bromine (Br2, a nonelectrolyte) is dissolved in 500 g of cyclohexane? (Kb for cyclohexane = 2.80 °C/m)

Answer 1

Apply Colligative properties

This is a typical example of colligative properties.

Recall that a solute ( non volatile ) can make a depression/increase in the freezing/boiling point via:

dTf = -Kf*molality * i

dTb = Kb*molality * i

where:

Kf = freezing point constant for the SOLVENT; Kb = boiling point constant for the SOLVENT;

molality = moles of SOLUTE / kg of SOLVENT

i = vant hoff coefficient, typically the total ion/molecular concentration.

At the end:

Tf mix = Tf solvent - dTf

Tb mix = Tb solvent - dTb

Tb mix = Tb solvent +   Kb*molality * i

i = 1 and molality = mass/MW /kg = (40/159.8080) / (0.5) = 0.50

Tb mix = (81) + (2.80)(0.50)

Tb mix = 82.4 C