Question

What mass of salt (NaCl) should you add to 1.40 L of water in an ice cream maker to make a solution that freezes at -11.2 ∘C ? Assume complete dissociation of the NaCl and density of 1.00 g/mL for water.        

Answer 1

i for NaCl is 2 since it breaks into Na+ and Cl-

use:

Δ Tf = i*Kf*mb

11.2 = 2.0*1.86 *mb

mb= 3.011 molal

volume of water is 1.40 L

density of water is 1.00 g/mL = 1.00 Kg/L

So, mass of water = 1.40 Kg

m(solvent)= 1.40 Kg

number of mol,

n = Molality * mass of solvent in Kg

= (3.011 molal)*(1.40 Kg)

= 4.215 mol

Molar mass of NaCl,

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

= 1*22.99 + 1*35.45

= 58.44 g/mol

mass of NaCl,

m = number of mol * molar mass

= 4.215*58.44

= 246 g

Answer: 246 g