Multiple Choice
Which aqueous solution has the lowest freezing point (assume all solutes are nonelectrolytes and the solutions are ideal)?
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14. Solutions
Freezing Point Depression
Multiple Choice
When 825 g of an unknown is dissolved in 3.45 L of water, the freezing point of the solution is decreased by 2.89°C. Assuming that the unknown compound is a non-electrolyte, calculate its molar mass.
A
154 g/mol
B
42.4 g/mol
C
44.5 g/mol
D
159 g/mol
Verified step by step guidance1
Identify the formula for freezing point depression: ΔT_f = i * K_f * m, where ΔT_f is the change in freezing point, i is the van't Hoff factor (which is 1 for non-electrolytes), K_f is the cryoscopic constant of the solvent (water in this case), and m is the molality of the solution.
Rearrange the formula to solve for molality (m): m = ΔT_f / (i * K_f). Substitute the given values: ΔT_f = 2.89°C, i = 1, and use the known K_f for water, which is 1.86°C kg/mol.
Calculate the molality of the solution using the rearranged formula. This will give you the number of moles of solute per kilogram of solvent.
Determine the number of moles of the unknown compound using the molality and the mass of the solvent. Since the solvent is water, convert the volume of water (3.45 L) to mass (in kg) using the density of water (1 kg/L).
Calculate the molar mass of the unknown compound by dividing the mass of the unknown (825 g) by the number of moles calculated in the previous step. This will give you the molar mass in g/mol.
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