Which of the following aqueous solutions has the lowest freezing point?
(A) 0.2 m NaCl
(B) 0.2 m CaCl2
(C) 0.2 m H2SO4
(D) 0.2 m NH3
(E) 0.2 m Al(NO3)3
Answer and Explanation:
Freezing point of a pure solvent depends on the amount of solute that gets dissolved in it. In order to determine which solution has the lowest freezing point, we need to look at the molality as well as whether the solute is ionic or covalent. Since the molality is equal for all of the answer choices, we need to look at what kind of solutes are present.
Choice A: NaCl is ionic since it has a metal and a nonmetal. NaCl dissociates into 2 ions: Na+ and Cl -
Choice B: CaCl2 is ionic since it has a metal and a nonmetal. CaCl2 dissociates into 3 ions: one Ca 2+ and two Cl- ions
Choice C: H2SO4 is a strong acid, and dissociates into 3 ions: two H+ and one SO4 2-
Choice D: NH3 is a covalent compound since it is composed of 2 nonmetals, and therefore doesn't dissociate when it is in water
Choice E: Al(NO3)3 is an ionic compound and will dissociate into 4 ions when it dissolves in water: one Al3+ and 3 NO3 - ions.
Thus, since Al(NO3)3 dissociates into the most ions, the solution with it will have lowest freezing point.