11
QuizMultiple Choice

Colligative Properties — Quiz

Question 11 • Chemistry

A group of students is in the laboratory performing an experiment on the effect of adding salt to water. Which observation are students likely to make as they increase the molality of the saltwater solution?

Answer
A
As the molality increases by 1 m , there is a corresponding increase of 1 point 8 6 times degree C in the solution’s freezing point.
B
As the molality increases by 1 m , there is a corresponding decrease of 1 point 8 6 times degree C in the solution’s freezing point.
C
As the molality increases by 1 m , there is an initial decrease of 1 point 8 6 times degree C followed by an increase of 1 point 8 6 times degree C in the solution’s freezing point.
D
As the molality increases by 1 m , there is no observable change in the solution’s freezing point.

Explanation

Dissolving salt in water lowers its freezing point, a colligative property. For water, the freezing-point depression is ΔT_f = iK_f m; for salt treated as one particle per formula unit, K_f is about 1.86 °C·kg/mol, so each 1 m increase in molality lowers the freezing point by about 1.86 °C. Therefore, the likely observation is a corresponding decrease, as stated in choice C.

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