Multiple Choice
Water has a molal freezing point constant of 1.86°C kg/mol. Calculate the freezing point of a solution containing 34.335 grams of calcium chloride (CaCl2) dissolved in 163.896 grams of water, assuming complete dissociation.
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14. Solutions
Freezing Point Depression
Multiple Choice
What is the freezing point of a solution that contains 50.0 g of FeBr₃ in 200.0 g of water, given that the freezing point depression constant (Kf) for water is 1.86 °C/m?
A
-3.72 °C
B
-5.58 °C
C
-1.86 °C
D
0.00 °C
Verified step by step guidance1
First, calculate the molar mass of FeBr₃. The molar mass is the sum of the atomic masses of iron (Fe) and bromine (Br). Use the periodic table to find these values: Fe = 55.85 g/mol, Br = 79.90 g/mol. Since there are three bromine atoms, the molar mass of FeBr₃ is: g/mol.
Next, determine the number of moles of FeBr₃ in the solution. Use the formula: . Substitute the mass of FeBr₃ (50.0 g) and the molar mass calculated in the previous step to find the moles.
Calculate the molality of the solution. Molality (m) is defined as moles of solute per kilogram of solvent. Use the formula: . Convert the mass of water (200.0 g) to kilograms and use the moles of FeBr₃ from the previous step.
Determine the van't Hoff factor (i) for FeBr₃. Since FeBr₃ dissociates into four ions (Fe3+ and 3 Br-), the van't Hoff factor is 4.
Finally, calculate the freezing point depression using the formula: . Substitute the values for Kf (1.86 °C/m), molality, and the van't Hoff factor to find the change in freezing point. Subtract this value from the normal freezing point of water (0.00 °C) to find the freezing point of the solution.
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