Which of the following dilute aqueous solutions has the highest freezing point?

0.1 M KNO3
0.2 M Na3PO4
0.25 M FeCl3
0.01 M Na2SO4

Correct answer: 0.01 M Na2SO4

Solution

Total effective concentrations: 0.1 M KNO3: 0.1*2=0.2; 0.2 M Na3PO4: 0.2*4=0.8; 0.25 M FeCl3: 0.25*4=1.0; 0.01 M Na2SO4: 0.01*3=0.03. Smallest effective concentration = 0.03 (Na2SO4), so it has the highest freezing point.

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