Choose the aqueous solution below with the highest boiling point. These are all solutions of nonvolatile solutes and you should assume ideal van't Hoff factors where applicable. A) 0.075 m NaI
B) 0.075 m (NH4)3PO4
C) 0.075 m NaBrO4
D) 0.075 m LiCN
E) 0.075 m KNO2

B. (NH₄)₃PO₄

Explanation:

Elevation of boiling point is one of the colligative properties.

Formula is: ΔT = Kb . m . i

ΔT = Boiling T° of solution - Boiling T° of pure solvent.

i = Van't Hoff factor (numbers of ions dissolved)

Kb = Ebulloscopic constant

In this case, all the m are the same so let's dissociate the salts:

NaI → Na⁺  +  I⁻     i = 2

(NH₄)₃PO₄ → 3NH₄⁺  +  PO₄⁻³     i = 4

NaBrO₄  →  Na⁺  +  BrO₄⁻    i = 2

LiCN  →  Li⁺  +  CN⁻         i = 2

KNO2  →  K⁺  +  NO₂⁻        i = 2

The biggest i would have the biggest boiling point, then B is the right answer, ammonium phosphate.

You count the i, with the moles of ions, you dissociate from the salt.