le contains an unknown potassium halide salt, KX. A student must determine the i
ID: 703366 • Letter: L
Question
le contains an unknown potassium halide salt, KX. A student must determine the identity of the salt by using freezing point depression data. Because KX is a strong electrolyte, one mole of KX yields two moles of ions in solution. The student finds that when 1.13 g of the unknown saltis dissolved in 24.65 g of distilled water, the freezing point of the resulting solution is-1.01 °C. The freezing point of the distilled water is determined to be+0.02 C a. Calculate the freezing point depression for the solution.
Explanation / Answer
Freezing point depression
Tf - Ts = i x Kf x m
Freezing point of water - freezing point of solution = van't Hoff factor x freezing point depression constant x Molality
0.02 - (-1.01) °C = 2 x 1.86 °C/m x mass of salt / (MW of salt x mass of water)
1.03 = 3.72 x 1.13 g /(M x 0.02465 kg)
0.0253586 M = 4.2036
M = 165.76 g/mol
Molecular weight of KX = 165.76
40 + X = 165.76
X = 126.67 = Molecular weight of Iodine
The halide should be KI
Freezing point depression of solution = Tf - Ts
= 0.02 - (-1.01) °C
= 1.03 °C
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