Question

Which of the following aqueous solutions will have the lowest freezing point?

	a. 0.04 m sodium sulfate, Na2SO4
	b. 0.08 m magnesium chloride, MgCl2
	c. 0.1 m potassium chloride, KCl
	d. 0.05 m sodium chloride, NaCl
	e. 0.1 m magnesium sulfate, MgSO4

Answer 1

Freezing point of water = 0 C

as all solution have water as solvent.

addition of salt will decrease the freezing point

which is given by

T = Kf * m * i

where Kf is freezing point depression constant and it's 1.86 C/m for water

m is the molality of solutioon

i is the Vant hoff factor ( number of dissolved particles)

we will find T for each solution

(a) sodium sulfate will disassociate completely into Na+ and SO₄^2- ions

Na₂SO₄ (aq) -----> 2 Na+ (aq) + SO₄^2- (aq)

so i = 3

T = 1.86 C/m * 0.04 m * 3 = 0.2232 C

(b) i = 3

T = 1.86 C/m * 0.08 m * 3 = 0.4464 C

(c) i =2

T = 1.86 C/m * 0.1 m * 2 = 0.372 C

(d) i =2

T = 1.86 C/m * 0.05 m * 2 = 0.186 C

(e) i = 2

T = 1.86 C/m * 0.1 m * 2 = 0.372 C

Frezing point is given by Tf(solution) = Tf(water) - T

so one with highest value of T will have lowest freezing point

so Answer is (b) 0.08 m of Magnesium chloride MgCl2