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 1.86 g of quinine is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 4.77 °C. The freezing point and Kf constant for cyclohexane can be found here. Calculate the molar mass of quinine.

Answer 1

Calculation of molar mass from the depression of freezing point:

constants=kf=20.1⁰c.kg/mole, (molal freezing point depression for cyclohexane), freezing point temp.=6.5⁰c, m=MOLALITY,

depression of freezing point is proportional to molar concentration, therefore,

6.50-4.77=1.73 (delta T)

Delta T=Kf*m, =

m=deltaT/Kf, 1.73/20.1=0.08606 MOLES OF SOLUTE/KG,

number of moles of solute=m*kgs of solvent,

=0.08606 per kg*0.025 (25.0 gm=0.025 kg),

=0.002151 number of moles of solute.   

molar mass= gms of solute/moles of solute=1.86/0.002151=864.71 gm/mol