Question

For a solution that contains 0.0216 kg of a nonelectrolyte and 0.175 kg of water. The solution froze at -7.18*C and Kf = 1.86*C/m. Is the nonelectrolyte CH3OH, CH3SH, or C2H5OH? Explain your reasoning.

Answer 1

use:

Δ Tf = Kf*mb

7.18 = 1.86 *mb

mb= 3.8602 molal

m(solvent)= 0.175 Kg

use:

number of mol,

n = Molality * mass of solvent in Kg

= (3.86 mol/Kg)*(0.175 Kg)

= 0.6755 mol

mass(solute)= 21.6 g

use:

number of mol = mass / molar mass

0.6755 mol = (21.6 g)/molar mass

molar mass = 31.97 g/mol

Lets find the molar mass of all the compounds given

Molar mass of CH3OH,

MM = 1*MM(C) + 4*MM(H) + 1*MM(O)

= 1*12.01 + 4*1.008 + 1*16.0

= 32.042 g/mol

Molar mass of CH3SH,

MM = 1*MM(C) + 4*MM(H) + 1*MM(S)

= 1*12.01 + 4*1.008 + 1*32.07

= 48.112 g/mol

Molar mass of C2H5OH,

MM = 2*MM(C) + 6*MM(H) + 1*MM(O)

= 2*12.01 + 6*1.008 + 1*16.0

= 46.068 g/mol

CH3OH has closest molar mass

Answer: CH3OH