Question 12.12: The osmotic pressure of a 0.010 M potassium iodide (KI) solu...

The osmotic pressure of a 0.010 M potassium iodide (KI) solution at 25°C is 0.465 atm. Calculate the van’t Hoff factor for KI at this concentration.

Strategy Note that KI is a strong electrolyte, so we expect it to dissociate completely in solution. If so, its osmotic pressure would be

2(0.010 M)(0.0821 L ⋅ atm/K ⋅ mol)(298 K) = 0.489 atm

However, the measured osmotic pressure is only 0.465 atm. The smaller than osmotic pressure means that there is ion-pair formation, which reduces the number of solute particles ( $K^{+} and I^{-} ions$ ) in solution.

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