Question

Use the model below to answer the question.

Answer 1

Answer:

C. Student A says adding a solute allows less water to leave (increases the boling point)

Explanation:

The figure (a) shows a pure solvent: in the surface there are only solvent particles and in the air surronding the solvent there are some solvent particles: those particles come from the solvent particles that are in the surface and reach enough energy to escape from the liquid phase and become gas particles.

The vapor pressure is the pressure of the molecules in the vapor phase in contact with the liquid phase.

The normal boiling point is temperature of the liquid at which the vapor pressure equals the atmospheric pressure.

The figure (b) shows a half-and-half mixture (solution) with a molecular (no ionic) non-volatile solute. Since the solution is homogeneous, now the surface of the liquied is occupied by solute and solvent particles (50/50). Thus, since there are less particles of solvent on the surface, the vapor pressure decreases, and the boiling point increases: there will be needed a higher temperature to evaporate the amount of solvent particles needed to equal the atmospheric pressure.

Analyze the statements:

A. Student B says adding an ionic solute increases the boiling point more than adding a covalent solute.

• This is not true because the model does not show a ionic solute but a molecular one.

B. Student D says adding a solute allows more water to leave (decreases the boiling point)

• This is not true because the model shows that adding a solute cause less water molecules to leave.

C. Student A says adding a solute allows less water to leave (increases the boiling point)

• This is the correct explanation: as explained above, when solute is added there are less molecules on the surface of the liquid and, consequently, less water molecules leave, which makes that the boiling point increases.

D. Student C says adding a solute has no effect on the boiling point

• This is false, as explained for the previous statement.