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Question:

Which of the following aqueous solutions has the highest boiling point?

0.1 M KNO3

0.1 M Na3PO4

0.1 M BaCl2

0.1 M K2SO4

Solution:

Boiling point elevation depends on the solvent, of course, but we're assuming water, and upon the concentration of the solute and how many particles into which the solute breaks. Colligative properties like boiling point elevation, depend on the total number of particles in solution. The solvent used here is water. So elevation in boiling point will be above a boiling point of water for all solutions. ΔT=i × Kb × m, where i= van Hoff's factor, Kb= boiling point elevation constant, m= molality. Higher ΔT will have a higher boiling point. For 0.1 M KNO3 (i) = (1 + (n-1)α) = 2 (α for salt is 1). Similarly for 0.1 M BaCl2 (i) = 3 (α for salt is 1). For 0.1 M Na3PO4 (i) = 4 (α for salt is 1). For 0.1 M K2SO4 (i) = 3 (α for salt is 1). Thus, Na3PO4 has the highest boiling point.