Question

6.42 M solution is prepared by dissolving each of these solutes to make separate solutions: ethanol (alcohol, C_2 2 ​ H_6 6 ​ O), barium chloride, ammonium phosphate, and calcium nitrate. Which pair of compounds gives the lowest and the highest boiling point elevation? [smallest elevation, first]

A. ethanol, magnesium chloride

B. magnesium chloride, calcium nitrate

C. ethanol, ammonium phosphate

D. calcium nitrate, ethanol

E. calcium nitrate, ammonium phosphate

Answer 1

Elevation in Boiling point = i*K_bm

Where m is the molality = 6.42m, which is same for all

K_b elevation constant, same for all

i = van't hoff factor = number of ions in solution

thus, Elevation in Boiling point i

A. ethanol , i= 1(non-electrolyte)

MgCl₂, i = 3(1Mg²⁺, 2Cl^-)

B. MgCl₂, i = 3

Ca(NO₃)₂, i = 3

Same in both cases

C. ethanol , i = 1

AlPO₄ = 2

D. Ca(NO₃)₂, i = 3

ethanol , i = 1

E. Ca(NO₃)₂, i = 3

AlPO₄ = 2

Thus, in A there is large difference between i i.e. 1 and 3

First one is smaller in this case and hence elevation will be smaller in this case and highest for latter case.

Thus, option A is correct