Which of these solutions has the highest boiling point?

Question

Which of these solutions has the highest boiling point?

	3 M CH4
	3 M SrF2
	3 M CaF
	3 M N2

Answer 1

The answer/explanation provided by @Brandon W would be appropriate to describe the boiling point of the individual compounds, but NOT to describe the boiling point of an aqueous solution containing these compounds. The boiling point of an aqueous solution is affected by the number of particles in solution (colligative property). Thus, we have the following situation:

CH₄ (non electrolyte) ==> CH₄ = 1 particle

SrF₂ ==> Sr²⁺(aq) + 2F^-(aq) = 3 paricles

CaF isn't a correct compound. It would be CaF₂ and would also have 3 particles

N₂ (non electrolyte) = N₂ = 1 particle

So, aqueous solutions of both SrF₂ and CaF₂ would have the same boiling points which would be higher than the boiling points of aqueous solutions of CH₄ and N₂.

Answer 2

Boiling point is affected by the strength of the force holding together particles of a substance. The stronger the force the higher the boiling point as more energy needs to be put in to break that force and allow the particles to separate which is what becoming a gas is. In the question, you have two nonpolar covalent compounds (CH4 and N2) being held together by London dispersion forces and two ionic compounds (SrF2 and CaF) being held together by ionic bonds. As ionic bonds are stronger than London dispersion forces by definition that immediately narrows down the possible answers to either SrF2 or CaF. To pick between them one must decide which ionic bond is stronger. The two factors which affect the strength of ionic bonds are ion size and ion charge. Sr and Ca both have charges of plus 2 but the Ca ion is smaller and since smaller ions have stronger ionic bonds one would expect CaF to have a higher boiling point.