Question

The freezing point depression constant of water is -1.86 C/m. If 7.50 g NaCl is dissolved in 45.0 g H, O, the freezing point is changed by -9.71 *C. Calculate the van't Hoff factor for Naci. Select one: O a. 1,00 . OOOO c. 1.83

Answer 1

Molar mass of NaCl,

MM = 1*MM(Na) + 1*MM(Cl)

= 1*22.99 + 1*35.45

= 58.44 g/mol

mass(NaCl)= 7.50 g

use:

number of mol of NaCl,

n = mass of NaCl/molar mass of NaCl

=(7.5 g)/(58.44 g/mol)

= 0.1283 mol

m(solvent)= 45.0 g

= 4.5*10^-2 kg

use:

Molality,

m = number of mol / mass of solvent in Kg

=(0.1283 mol)/(4.5*10^-2 Kg)

= 2.852 molal

Kf for water is 1.86 oC/m

Use:

Delta Tf = i*Kf*m

9.71 = i*1.86*2.852

i = 1.83

Answer: c