Boiling Point elevation: Intermolecular Forces vs Van't Hoff Factors?

[kevin2400]

I was taking one of the berkeley review cbt's and the question was asking which of the molecules had a higher boiling point.

Well at the moment my thought process was the higher the intermolecular forces (hydrogen bonding), the higher the boiling point, so I chose the answer with hydrogen bonding, ethanol.

But looking at the solutions, the answer was Magnesium Chloride, and the reasoning was because when Magnesium Chloride dissociates into water, it has a Vant Hoff factor of 3, (so in the equation k*i*m, MgCl2 would have a greater value of i, and thus a higher boiling point) which is more than the ethanol's Vant Hoff factor.

So my question is, when you go about finding out which one has the highest boiling point, which is the correct way to think about it? Would you go by intermolecular forces (hydrogen bonding, dipoles, london disperson forces) or would you go by the k*i*m equation? Do they ever contradict each other?


Edit: So I forgot to mention that this was based on a passage (which talked about bp elevation and gave the equation change in boiling point = k*i*m within the passage), not a free standing question. I suppose that makes sense since the passage gave the equation, so maybe that was the way they expected?

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[going2breakdown]

This is a question about colligative properties. Remember what the colligative properties are: freezing point depression, boiling point elevation, vapor pressure lowering and osmotic pressure. When you add the salt all colligative properties change. (i.e. boiling point is raised AND vapor pressure is lowered, AND freezing point dep. occurs etc. Save thought by remembering that).
Your question involves boiling point. The boiling point is a colligative property, period. When you add the solute to the solvent think; all that matters now is the number of molecules in the solvent (Vant Hoff factor), regardless of identity. Also the solvent will be the same if they ask you to compare the boiling point properties.
You are a little confused about what they will ask you. They might want you to compare something like the boiling point of ether compared to the boiling point of some alcohol. They will be talking about the boiling point of a PURE substance. There should be a list in your books about the general trend in boiling points (e.g. alcohol vs. phenols etc.). Don't dwell on this too much, just remember; boiling point is a colligative property, and for all intents and purposes they are going to be referring to some SOLUTE IN A SOLVENT when asking you. Unless the substance is pure just remember it's a colligative property.
Double check this with someone else. I believe this is correct though. Good luck.