The boiling point and freezing points of a solution differs from those of the pu

Question

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, ?Tb, can be calculated using the equation

?Tb=Kb?m

in which m is the molality of the solution and Kbis the molal boiling-point-elevation constant for the solvent. The freezing-point depression, ?Tf, can be calculated in a similar manner using

?Tf=Kf?m

in which m is the molality of the solution and Kfis the molal freezing-point-depression constant for the solvent.

What is the freezing point of radiator fluid that is 50% antifreeze by mass?
Kf for water is 1.86 ?C/m.

Express your answer to three significant figures and include the appropriate units.

What is the boiling point of radiator fluid that is 50% antifreeze by mass?
Kb for water is 0.512 ?C/m.

Express your answer to one decimal place and include the appropriate units.

Explanation / Answer

Answer

Radiator fluid is ethylene glycol and its molecular weight is 62.07 g/mol

50% by mass of this in water corresponds to a molality {(50/62.07)/50}*1000 = 16.11 m

delta Tf = 1.86*16.11 = 29.966C and the freezing point of radiator fluid is -29.966 oC

delta Tb = 0.512*16.11 = 8.249 and the boiling point of radiator fluid is 100 + 8.249 = 108.249 oC

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