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 3.28 g of quinine is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 8.43 °
c. The freezing point and Kf constant for cyclohexane can be found here.
Calculate the molar mass of quinine.

Answer 1

For the answer to the question above, The formula for freezing point depression is 
ΔTf = mkfi 

kf is the freezing point constant 
i is the Van't Hoff factor which in this case is 1 
m is molality (moles of solute/kg of solvent) 
ΔTf is temperature change 

ΔTf is 2.17 °C, the molality is the amount of solute Quinine 
in the solvent cyclohexane. We cannot calculate moles therefore we need to substitute moles with g/mm. 

moles = g/mm so molality=(g/mm)/kg 
molality = (0.845/mm)/0.025 = 33.8/mm 

2.17 = 33.8/mm(20.8) rearrange 

mm = (33.8/2.17)(20.8) = 324g/mol