Question

Quinine is a natural product extracted from the bark of the cinchona tree, which is native to South America. Quinine is used as an antimalarial agent. When 2.03 g of quinine is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 5.20 °C. The freezing point and Kf constant for cyclohexane can be found here. Calculate the molar mass of quinine.

Answer 1

Given that mass of quinine w = 2.03 g

molar mass of quinine M = ?

mass of solvent (cyclohexane ) W = 25 g = 0.025 kg

Kf of cyclohexane = 20.2 ^oC/m

depression in freezing point dT =  5.20 °C

We know that

   depression in freezing point ΔT = Kf x molality

molality = (w/M) x (1/W in kg)

Then, depression in freezing point ΔT = Kf x { (w/M) x (1/W) }

M = (Kf x w ) / (ΔT x W)

=  (20.2 ^oC/m x 2.03 g) / (5.2 ^oC x  0.025 kg)

= 315.4

M = 315.4

Therefore, molar mass of quinine = 315.4 g/mol