Question

1.512 g of non electrolyte nitrobenzene(123 g/mol) is dissolved in 50 g of chloroform(CHCl3), the mixture freezes at -64.65 degrees celcius. Find Kf of chloroform if the pure solvent freezes at -63.5 degrees celcius.

Answer 1

Given: Molar mass of nitrobenzene , M = 123g/mol

Mass of nitrobenzene added , m = 1.512g

Mass of chloroform = 50g = (50g/1000g)*1kg = 0.05kg

Freezing point of mixture , T_mix = -64.65^oC

Freezing point of pure solvent, T_solvent = -63.5^oC

Step 1: To calculate change in temperature, T_f :

  T_f = T_solvent- T_mix = -63.5 - (-64.65)

T_f = 1.15^oC

Step 2: To calculate molality,m

Number of moles of nitrobenzene,n = mass of nitrobenzene,m / molar mass of nitrobenzene,M

n = 1.512/123 = 0.01229 mol

molality = no. of moles of nitronezene / kg of chloroform

molality,m = 0.01229mol/0.05kg = 0.2458 m

Step 3 : To calculate k_f

  T_f = k_f m

1.15^oC = k_f (0.2458m)

k_f = 1.15^oC / 0.2458m

k_f = 4.678 ^oC/m