Multiple Choice
Which of the following statements correctly describes the effect of adding a nonvolatile solute to a solvent according to Raoult's Law?
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14. Solutions
Vapor Pressure Lowering (Raoult's Law)
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The vapor pressure of water at 100.0ºC is 0.720 atm. Determine the mass percent of iron (II) chloride, FeCl2, needed to reduce its vapor pressure to 0.655 atm. (MW of FeCl2 is 126.756 g/mol)
A
67.7%
B
18.9%
C
22.5%
D
58.3%
Verified step by step guidance1
Understand that the problem involves a colligative property, specifically vapor pressure lowering, which is described by Raoult's Law.
Raoult's Law states that the vapor pressure of a solvent above a solution (P_solution) is equal to the vapor pressure of the pure solvent (P_solvent) times the mole fraction of the solvent (X_solvent): P_solution = P_solvent * X_solvent.
Calculate the mole fraction of water (X_water) using the given vapor pressures: X_water = P_solution / P_solvent = 0.655 atm / 0.720 atm.
Determine the mole fraction of FeCl2 (X_FeCl2) using the relationship: X_water + X_FeCl2 = 1.
Convert the mole fraction of FeCl2 to mass percent. First, calculate the moles of FeCl2 using its molar mass (126.756 g/mol), then find the mass of FeCl2 and water, and finally calculate the mass percent: (mass of FeCl2 / (mass of FeCl2 + mass of water)) * 100%.
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