Question

Which of the following aqueous solutions will have the highest vapor pressure at 25 degrees Celsius?

1.0 M C6H12O6
1.0 M LiNO3
1.0 M Al2O3
1.0 M CaF2

Answer 1

I am going with C, but I'm not sure

Answer 2

@Cuanchi

Answer 3

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.

Answer 4

nicely explained @rushwr

Vapor-pressure has to do with intermolecular forces, if something has weaker IMF it's more likely to go from (liquid)--(gas) phase.

but yeah, if something is dissolved in a solution it's called colliagitive properties, meaning that it depends on how much rather than the identity of the solute that you dissolve in the solvent.

Answer 5

I think that's the van't Hoff factor, i which is the number of moles of ions produced by a compound dissolved in a solute, it's used in the freezing point depression so you write the balanced equation and add up the moles of ions produced.

Answer 6

Thank you so much for these explanations! ;)