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 1.53 g of quinine is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 3.93 °c. the freezing point and kf constant for cyclohexane can be found here. calculate the molar mass of quinine.

The formula for freezing point depression is: ΔTf = mkfi 

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

ΔTf is 3.93°C, the molality is the amount of solute Quinine in the solvent cyclohexane. Moles cannot be calculated, therefore substitution of moles with g/mm is needed.
moles is g/m, therefore molality is equal to (g/mm)/kg 

Calculations:

molality = (0.153/mm)/0.025 = 6.12/mm 

3.93 = 6.12/mm(20.8)

mm = (6.12/3.93)(20.8) = 32.39 g/mol

there are two significant figures.