Vapor Pressure Depression Van Hoff Factor

[CANgnome]

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So, I do not have to include the i value for calculation if the solute dissociated into the solvent, right?

1 mole NaCl in 1 mole H2O

Would the mole fraction be 1/2 or 1/3?

VP depression IS a colligative property, and all the others, such as osmotic pressure, bp elevation, mp depression all include (i), so should VP depression? I don't think I have actually hit such a problem before.


My guess is no.

 

[Troyvdg]

So, I do not have to include the i value for calculation if the solute dissociated into the solvent, right?

1 mole NaCl in 1 mole H2O

Would the mole fraction be 1/2 or 1/3?

VP depression IS a colligative property, and all the others, such as osmotic pressure, bp elevation, mp depression all include (i), so should VP depression? I don't think I have actually hit such a problem before.


My guess is no.

no, there's no van hoff factor you have to include.

the mole fraction is moles of solute/total moles

 

[CANgnome]

Thx, good to confirm.

I think it is just getting too close to test day and I am becoming paranoid about everything 😀

 

[gigawatt]

1/3.

Mole fractions already count individual, dissociated particles so there's no need for an additional van't Hoff factor. That's the purpose of i in those other equations, after all, is to count individual, dissociated solute particles rather than "whole", undissociated solute particles.

 

[CANgnome]

1/3.

Mole fractions already count individual, dissociated particles so there's no need for an additional van't Hoff factor. That's the purpose of i in those other equations, after all, is to count individual, dissociated solute particles rather than "whole", undissociated solute particles.

er, so for 1 mole of NaCl in 1 mole of H2O

It raoult's law for new partial pressure of water would be:


1 mole H2O / (1 mole H2O + 1 mole Na+ + 1 mole Cl-) = 1/3

???

 

[CANgnome]

Aaaaaah

So i was correct to have suspected something.

it is 1/3, and not 1/2


it is the moles of the solvent / moles of solvent + moles of ions


this means that NaCl actually +2 moles to the overall!


(i googled raoult nacl and found a pdf)