Question

Which of the following four aqueous solutions will have the lowest freezing point? i) 0.05 m sucrose ii) 0.05 m CaCl2 iii) 0.05 m NaCl iv) 0.05 m AlCl3

MgCl2 is widely used to prevent ice formation on roadways. If this solution has non-ideal behavior, the van’t Hoff factor for a 0.05 m aqueous solution of MgCl2 would reasonably be _____. a. 1 b. 2 c. 2.7 d. 3 e. 3.2

Assuming ideal behavior, the change in the freezing point (deltaTf) for a 0.171m solution of NaCl in an unknown polar solvent is found to be −6.81 °C. Again, assuming ideal behavior, the change in the freezing point for a 0.684 m solution of NaCl in the same polar solvent will be ________.

a. −27.24 °C b. −6.81 °C c. +6.81 °C d. −54.48 °C e. −13.62 °C

Answer 1

Given aqueous solutions are having the same concentration that is 0.05m.

i) 0.05 m sucrose:

Sucrose is a covalent molecule, hence it won't undergo dissociation.

So there is no change in concentration.

ii) 0.05 m CaCl2

CaCl2(aq) -> Ca⁺² (aq)+ 2Cl^-(aq)

Hence, three ions per molecule are produced.

iii) 0.05 m NaCl

NaCl(aq) -> Na⁺(aq) + Cl^-(aq)

Hence, two ions per molecule are produced.

iv) 0.05 m AlCl3

AlCl_3(aq) -> Al⁺³(aq) +3Cl^-(aq)

Hence, four ions per molecule are produced.

Lowering in freezing point depends on the number of solute particles.

option iv) 0.05m AlCl3 will have lowest freezing point.

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MgCl2 undergoes dissociation in aqueous solution.

MgCl2(aq) -> Mg⁺² (aq)+ 2Cl^-(aq)

vont hoff factor i = 1+ alpha(n-1) ----------------(1)

where alpha =degree of dissociation (in case of ionic compound, alpha=100%)

n=number of ions produced on dissociation

Substitute in (1), we get the value of i.

i=1+ (3-1)

therefore i=3

option d is the correct answer.

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deltaTf = Kf x molality

Given deltaTf1 =−6.81 °C

molality1 = 0.171m

When molality2 = 0.684 m

What is the value of deltaTf2 ?

Kf is constant for a particular solvent. So deltaTf1 /molality1 = deltaTf2/molality2

=> −6.81 °C/ 0.171m = deltaTf2/0.684 m

=> deltaTf2 = (−6.81 °C/ 0.171m ) . 0.684 m

                   = -27.24°C

Hence, option a is the correct answer.