MCAT Chemistry Question — Aqueous Solution Boiling Point

This question is testing your understanding of colligative properties as they relate to boiling-point elevation. The change in temperature for the boiling point of a solution follows the equation ΔT_b = K_bmi, where K_b represents the boiling point elevation constant (for water, Kb = +0.52^oC/m), m represents the molality of the solution (moles of solute/kg of solvent), and i represents the van’t Hoff factor.

Since the boiling point elevation is proportional to the molality multiplied by the van’t Hoff factor, you must consider each variable in the overall determination. Upon hydration of each sodium phosphate, three Na⁺ ions and one PO₄^3- ion are produced, giving a van’t Hoff factor of 4. Overall, ΔT_b = K_b(0.1)(4) = 0.4K_b. For sodium chloride, one Na⁺ ion and one Cl^– ion are produced, yielding a ΔT_b = K_b(0.15)(2) = 0.3K_b. Since hydrogen fluoride is a weak electrolyte (weak acid), i=1 and ΔT_b = K_b(0.2)(1) = 0.2K_b. Lastly, methanol is a non-electrolyte and does not dissociate in solution, resulting in a van’t Hoff factor of one and ΔT_b = K_b(0.3)(1) = 0.3K_b. Thus, sodium phosphate will have the highest boiling point making A the correct answer.