Question

A student prepares four 0.10 molal aqueous solutions, each containing one of the solutes given below. Which solution has the lowest freezing point? K[Fe(CN) KOH CH3COOH NHANO​

Answer 1

I don't know so I can't solve it

Answer 2

Given info : A student prepares four 0.10 molal aqueous solutions, each containing one of the solutes given below -

K₄[Fe(CN)₆] , KOH , CH₃COOH , NH₄NO₃

To find : which solution has the lowest freezing point ?

solution : depression in freezing point is given by, ∆Tf = i × Kf × n

where ∆Tf is change in freezing point, i is Vant Hoff factor , Kf is freezing point constant and m is molality of solution.

here in question, all solutions have same molality and also solutes contain in same solvent (i.e., aqueous solution)

so, m and Kf are constants.

∴ ∆Tf ∝ i

it means change in freezing point depends only upon Vant Hoff factor.

if we assume all electrolytes are completely dissociates.

then, degree of disassociation, α = 1

now for K₄[Fe(CN)₆] ⇒4K⁺ + [Fe(CN)₆]⁴¯

i = 1 - α + 4α + α = 1 + 4α = 1 + 4 = 5

for KOH ⇒K⁺ + OH¯

i = 1 - α + α + α = 1 + 1 = 2

for CH₃COOH ⇒CH₃COO¯ + H⁺

i = 1 - α + α + α = 1 + α = 1 + 1 = 2

for NH₄NO₃ ⇒NH₄⁺ + NO₃¯

i = 1 - α + α + α = 1 + α = 1 + 1 = 2

now it is clear that value of i for K₄[Fe(CN)₆] is maximum. so change in freezing point will be maximum for K₄[Fe(CN)₆]. it means K₄[Fe(CN)₆] lower the freezing point to the maximum value. hence freezing point will be minimum for K₄[Fe(CN)₆]