Question

Determine the colligative (effective) molality of a truly saturated salt solution and the freezing point of a truly saturated salt solution (solubility limit of NaCl is 35.7 g)

Answer 1

Colligative molality of a truly saturated salt solution = [no.of moles of salt / weight of the solvent in grams] x 1000

no.of moles of salt solute = given weight of salt/ its molar mass

                                              = 35.7g/58.5g/mol

                                               = 0.61mol

Given solubility value. Solubility is measured for 100g of solvent.

Hence molality = (0.61 moles/ 100g) x 1000

                            = 6.1m

freezing point of truly saturated salt solution :

freezing ponit of pure water = 0⁰C

depression in freezing point of salt solution = Kf x molality of solution.

K_f is 1.853 K·kg/mol. for saturated salt solution.

273K - Tf = 1.853 K·kg/mol x 6.1molal

Tf = 273 K - 11.3K

    = 261.7 K

HENCE THE FREEZING POINT OF THE SOLUTION IS :

-11.3⁰C