Open Question
A 0.95 m aqueous solution of an ionic compound with the formula MX has a freezing point of -3.0 °C. Calculate the van’t Hoff factor (i) for MX at this concentration.
14. Solutions
Freezing Point Depression
Multiple Choice
A solvent has a freezing point of 10.1°C and a freezing point depression constant of 6.16°C/m. If a solution, made by dissolving 1.218 g of an unknown solute into 25.0 g of the solvent, has a freezing point of 8.8°C, what is the molar mass of the solute?
A
110.3 g/mol
B
92.1 g/mol
C
58.5 g/mol
D
75.0 g/mol
Verified step by step guidance1
Identify the change in freezing point by subtracting the freezing point of the solution from the freezing point of the pure solvent: ΔT_f = 10.1°C - 8.8°C.
Use the formula for freezing point depression: ΔT_f = i * K_f * m, where i is the van't Hoff factor (assumed to be 1 for non-electrolytes), K_f is the freezing point depression constant, and m is the molality of the solution.
Calculate the molality (m) using the formula: m = ΔT_f / K_f. Substitute the values for ΔT_f and K_f to find the molality.
Determine the number of moles of solute using the formula: moles = molality * mass of solvent (in kg). Convert the mass of the solvent from grams to kilograms before substituting.
Calculate the molar mass of the solute using the formula: molar mass = mass of solute (in grams) / moles of solute. Substitute the mass of the solute and the moles calculated in the previous step.
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