Multiple Choice
What mass in grams of NaCl would need to be added to 2051 g of water to increase the boiling temperature of the solution by 1.500 °C? (Kb for water is 0.5100 °C/m)
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14. Solutions
Boiling Point Elevation
Multiple Choice
Which of the following best describes the effect of adding a nonvolatile solute to a solvent on its boiling and freezing points?
A
Both the boiling point and freezing point increase.
B
Both the boiling point and freezing point decrease.
C
The boiling point increases and the freezing point decreases.
D
The boiling point decreases and the freezing point increases.
Verified step by step guidance1
Understand that adding a nonvolatile solute to a solvent causes colligative property changes, which depend on the number of solute particles, not their identity.
Recall that the boiling point elevation occurs because the solute lowers the solvent's vapor pressure, requiring a higher temperature to reach atmospheric pressure. This is described by the formula: \(\Delta T_b = i K_b m\), where \(\Delta T_b\) is the boiling point elevation, \(i\) is the van't Hoff factor, \(K_b\) is the ebullioscopic constant, and \(m\) is the molality of the solution.
Recognize that the freezing point depression happens because the solute disrupts the formation of the solid phase, lowering the temperature at which the solvent freezes. This is given by: \(\Delta T_f = i K_f m\), where \(\Delta T_f\) is the freezing point depression, \(K_f\) is the cryoscopic constant, and \(m\) is the molality.
Combine these concepts to conclude that adding a nonvolatile solute increases the boiling point and decreases the freezing point of the solvent.
Therefore, the correct description is that the boiling point increases and the freezing point decreases upon addition of a nonvolatile solute.
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