Question

Which aqueous solution of KI freezes at the lowest temperature?

a. 1 mol of KI in 500. g of water

b. 2 mol of KI in 500. g of water

c. 1 mol of KI in 1000. g of water

d. 2 mol of KI in 1000. g of water

Answer 1

b. 2 mol of KI in 500. g of water

Step-by-step explanation:

We have to apply the colligative property of freezing point depression.

The formula is: ΔT = Kf . m . i

As the (Kf . m . i) is higher, then the freezing temperature will be lower.

i refers to the Van't Hoff factor (number of ions dissolved in the solution)

KI → K⁺ + I⁻ (i =2)

Kf is constant so, we have to search for the highest m (molality)

Molality means the moles of solute in 1kg of solvent.

The highest m is option b → 2 mol of KI / 0.5 kg = 4 mol/kg

a. 1 mol of KI / 0.5 kg = 2 mol/kg

c. 1 mol of KI / 1kg = 1 mol/kg

d. 2 mol of KI / 1kg = 2 mol/kg

1000 g = 1kg. In order to determine molality we need to convert the mass (g) of solvent to kg