How do I figure out boiling point?

Question

A student experimentally determines that a 1M aqueous MgCl₂ solution boils at 115°C. He then decides to determine the boiling point of a 1M LiF solution. Based on his experiment, which of the following represents a potential boiling point of a 1M LiF solution? 

A. 108c

B. 118c

C. 98c

D. 100c

Answer 1

MgCl2 dissociates into 1Mg²⁺ and 2 Cl^1- ions for a total of 3 particles. At a boiling point of 115°C, it has a boiling point elevation of 15°C.

LiF dissociates into 1 Li¹⁺ and 1 F^1- ion for a total of 2 particles. On the basis of colligative properties, the boiling point elevation would be expected to be 2/3 of the elevation for MgCl2. This calculates to 10°C. The closest boiling point corresponding to this elevation is A at 108°C.