van 't Hoff Factor Problems

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1) The freezing point depression of a 0.10 m solution of HF(aq) solution is -0.201 °C. Calculate the percent dissociation of HF(aq).

Solution

The freezing point depression equation is:

ΔT = i Kf m

The only value we do not know is i:

0.201 °C = (x) (1.86 °C m¯1 ) (0.10m)

x = 1.08

The HF is about 8% dissociated.


2) A 1.00 m solution of acetic acid (CH3COOH) in benzene has a freezing point depression of 2.6 K. Calculate the value for i and suggest an explanation for its value.

Solution

The freezing point depression equation is:

ΔT = i Kf m

The only value we do not know is i:

2.6 K = (x) (5.10 K m¯1 ) (1.00m)

x = 0.51

I'd like you to think about the answer. If i = 1, then one item dissolving yields one item in solution. If i = 2, then one item dissolving yields two items in solution. What about i = 0.5?

Go to the answer.

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