Colligative Properties — Quiz

Question 30 • Chemistry

Which graph correctly shows the effect on the freezing point caused by increasing the molality of a solution ?

Answer

Image with description A graph has a horizontal axis labeled as molality and a vertical axis labeled as F subscript P. The horizontal axis ranges from negative 2 to 6 in increments of 1 and the vertical axis ranges from negative 9 to 2 in increments of 1. The graph has five plotted points at (0 comma 0), (1 point 0 comma negative 1 point 8), (2 point 0 comma negative 1 point 8), (3 point 0 comma negative 1 point 8), and (4 point 0 comma negative 1 point 8).

Image with description A graph has a horizontal axis labeled as molality and a vertical axis labeled as F subscript P. The horizontal axis ranges from negative 2 to 6 in increments of 1 and the vertical axis ranges from negative 9 to 2 in increments of 1. The graph has five plotted points at (0 comma 0), (1 point 0 comma negative 1 point 8), (2 point 0 comma negative 3 point 8), (3 point 0 comma negative 1 point 8), and (4 point 0 comma negative 3 point 8).

Image with description A graph has a horizontal axis labeled as molality and a vertical axis labeled as F subscript P. The horizontal axis ranges from negative 2 to 6 in increments of 1 and the vertical axis ranges from negative 9 to 2 in increments of 1. The graph has five plotted points at (0 comma 0), (0 point 9 comma negative 7 point 5), (2 point 0 comma negative 5 point 6), (2 point 9 comma negative 3 point 7), and (3 point 9 comma negative 1 point 8).

Image with description A graph has a horizontal axis labeled as molality and a vertical axis labeled as F subscript P. The horizontal axis ranges from negative 2 to 6 in increments of 1 and the vertical axis ranges from negative 9 to 2 in increments of 1. The graph has five plotted points at (0 comma 0), (1 point 0 comma negative 1 point 8), (2 point 0 comma negative 3 point 8), (3 point 0 comma negative 5 point 4), and (4 point 0 comma negative 7 point 2).

Explanation

Freezing-point depression increases with molality: ΔT_f = K_f m, so the freezing point becomes progressively lower as molality rises. The graph should therefore start at 0 when molality is 0 and show a steadily decreasing freezing point. Choice D shows that trend, dropping from 0 °F to −7.2 °F as molality increases from 0 to 4.

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