The boiling point and freezing points of a solution differs from those of the pure liquid. This can be explained in terms of vapor pressure. Since the vapor pressure of the solvent above the solution is lower, a higher temperature is needed to reach a point where the vapor pressure of the liquid meets the required 1 atm, and the boiling point is elevated. The lower vapor pressure changes the entire phase diagram for the solvent, and the resulting change pushes the triple point of the solution to a lower temperature value. The solid-liquid phase equilibria curve is related to the location of the triple point, and the freezing point is also lower.
The change in the boiling point for a solution containing a molecular solute, ÎTb, can be calculated using the equation
ÎTb=Kbâ m
in which m is the molality of the solution and Kb is the molal boiling-point-elevation constant for the solvent. The freezing-point depression, ÎTf, can be calculated in a similar manner using
ÎTf=Kfâ m
in which m is the molality of the solution and Kf is the molal freezing-point-depression constant for the solvent.
Ethylene glycol, the primary ingredient in antifreeze, has the chemical formula C2H6O2. The radiator fluid used in most cars is a half-and-half mixture of water and antifreeze.
Part A
What is the freezing point of radiator fluid that is 50% antifreeze by mass?
Kf for water is 1.86 âC/m.
Express your answer to three significant figures and include the appropriate units.
Part B
What is the boiling point of radiator fluid that is 50% antifreeze by mass?
Kb for water is 0.512 âC/m.
Express your answer to one decimal place and include the appropriate units.
The boiling point and freezing points of a solution differs from those of the pure liquid. This can be explained in terms of vapor pressure. Since the vapor pressure of the solvent above the solution is lower, a higher temperature is needed to reach a point where the vapor pressure of the liquid meets the required 1 atm, and the boiling point is elevated. The lower vapor pressure changes the entire phase diagram for the solvent, and the resulting change pushes the triple point of the solution to a lower temperature value. The solid-liquid phase equilibria curve is related to the location of the triple point, and the freezing point is also lower.
The change in the boiling point for a solution containing a molecular solute, ÎTb, can be calculated using the equation
ÎTb=Kbâ m
in which m is the molality of the solution and Kb is the molal boiling-point-elevation constant for the solvent. The freezing-point depression, ÎTf, can be calculated in a similar manner using
ÎTf=Kfâ m
in which m is the molality of the solution and Kf is the molal freezing-point-depression constant for the solvent.
Ethylene glycol, the primary ingredient in antifreeze, has the chemical formula C2H6O2. The radiator fluid used in most cars is a half-and-half mixture of water and antifreeze.
Part A
What is the freezing point of radiator fluid that is 50% antifreeze by mass?
Kf for water is 1.86 âC/m.
Express your answer to three significant figures and include the appropriate units.
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Part B
What is the boiling point of radiator fluid that is 50% antifreeze by mass?
Kb for water is 0.512 âC/m.
Express your answer to one decimal place and include the appropriate units.
| | ||
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