Question

PLEASE

Which of the following aqueous solutions will have the lowest vapor pressure at 25°C?

Answer 1

1.5 M C6H12O6

1.5 M LiNO3

1.0 M Al2O3

1.0 M CaF2

Answer 2

Vapour pressure lowering is a colligative property, one that depends on the number, rather than identity, of dissolved solutes. Dissolved solutes lower the vapour pressure of a liquid since they interfere with the conversion of liquid to gas at the liquid-gas interface. Therefore, the more dissolved solutes you have (i.e. the greater the concentration), the lower the vapour pressure is since the transition from liquid to gas is more obstructed.

C6H12O6 is glucose (or several other sugars, doesn't really matter), which does not dissociate into additional ions when dissolved. Therefore 1.5 M C6H12O6 produces 1.5 M of solute.

LiNO3 dissociates into Li+ and NO3- when dissolved, so every mole of LiNO3 produces 2 moles of dissolved solute. Therefore, 1.5 M LiNO3 produces 3 M of solute.

Al2O3 dissociates into 2 Al3+ and 3 O2- when dissolved, so every mole of Al2O3 produces 5 moles of dissolved solute. Therefore, 1.0 M Al2O3 produces 5 M of solute.

CaF2 dissociates into Ca2+ and 2 F- when dissolved, so every mole of 1.0 M Al2O3 produces 3 moles of dissolved solute. Therefore, 1.0 M CaF2 produces 3 M of solute.

Of these four choices, 1.0 M Al2O3 produces the greatest concentration of dissolved solute, so it will lower vapour pressure most. Hope that helps!