MCAT Chemistry Question — Aqueous Solution Boiling Point
- Apr 07, 2017
- MCAT Question of the Day
- 0.10 m Na3PO4
- 0.15 m NaCl
- 0.20 m HF
- 0.30 m CH3OH
Click for Explanation
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 PO43- ion are produced, giving a van’t Hoff factor of 4. Overall, ΔTb = Kb(0.1)(4) = 0.4Kb. For sodium chloride, one Na+ ion and one Cl– ion are produced, yielding a ΔTb = Kb(0.15)(2) = 0.3Kb. Since hydrogen fluoride is a weak electrolyte (weak acid), i=1 and ΔTb = Kb(0.2)(1) = 0.2Kb. Lastly, methanol is a non-electrolyte and does not dissociate in solution, resulting in a van’t Hoff factor of one and ΔTb = Kb(0.3)(1) = 0.3Kb. Thus, sodium phosphate will have the highest boiling point making A the correct answer.
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