A quantity of 3.125 g of a mixture of KCl and NaCl dissolved in 1 kg of water produces a depression

Question

A quantity of 3.125 g of a mixture of KCl and NaCl dissolved in 1 kg of water produces a depression of 0.186°C in freezing point. The molar ratio of KCl to NaCl in the solution (assuming complete dissociation of the salts) is (Kf = 1.86 deg/molal)

Accepted Answer

Let the mixture contain x mole KCl and y mole NaCl. Then

$x \times 74.5 + y \times 58.5 = 3.125(1) $$$\text{and }(x + y) \times 2 = \frac{\Delta Tf}{K_{f}} = \frac{0.186}{1.86} = 0.1(2)$$

$\therefore\frac{x}{y} = \frac{1}{3}$

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