Chemistry Question

Question

The magnitude of boiling point elevation is _________ the freezing point depression for a certain concentration of solute in the aqueous solution.

Always less than

Sometimes equal to

Always greater than

Always equal to 

Answer 1

This question is touching on the concept of colligative properties. Both boiling point elevation and freezing point depression are proportionally related to the solutes dissolved (Raout's Law). However, there is a difference in the actual changes due to differences in the solvent constants. Let's look at the two equations to see where the difference comes into play:

Boiling point elevation: DeltaT = change in temperature; K_b = molal boiling point elevation constant (different for each solvent); m = molal concentration of solute.

DeltaT = K_b*m

Freezing point depression: K_f = molal freezing point depression constant

DeltaT = K_f*m

K_b and K_f vary depending on the solvent; however, for water and most solvents K_f is greater than K_b meaning there will be a greater magnitude of change in freezing point depression than magnitude of freezing point depression.

The most likely answer is: The magnitude of boiling point elevation is "always less than" the freezing point depression for a certain concentration of solute in the aqueous solution.