Question

Which one of the following aqueous solutions would have the lowest freezing point? Explain your reasoning.

0.030M HCl 0.025M Mg(NO3)2 0.020M BaCl2 0.025 M HC2H3O2

Answer 1

0.030M HCl on dissociation gives two ions

0.025M Mg(NO₃)₂ on dissociation gives three ions

0.020M BaCl₂ on dissociation gives three ions

0.025 M HC₂H₃O₂ on dissociation gives only one ion (weak acid)

Freezing point lowering is a colligative property, so its magnitude depends on the number of particles (ions or molecules) in a given volume of the solution. The given aqueous solutions of the ionic salts dissociates in solution, we get 2, 3, 3 and 1 ions, respectively. 0.025M Mg(NO₃)₂  and 0.020M BaCl₂ both have same number of ions on dissociation, so let us consider the molality. Molality of 0.025M Mg(NO₃)₂ is more (freezing point lowering α molality). Therefore 0.020 M BaCl₂ will have the lowest freezing point.