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During the lecture in my chemistry class, my teacher posed the following problem:

Arrange in terms of increasing vapor pressure:

a) 0.5m sugar

b) 0.5m NaCl

c) 2m ethylene glycol

d) 2m LiBr

The answer turned out to be: d, c, b, a. However, I do not understand why this is the case. I know the formula for vapor pressure is:

$ \Delta P = -XP° $

...which means that the pressure of the solvent is proportional to the mole fraction of the solute. I'm still not grasping why the aforementioned answer is correct though. Can anyone provide me with an explanation?

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You must've been given more information. What kind of data have you been given on these substances? Vapor pressure is related to the enthalpy of vaporization as far as I know. Similarly, the mole fraction of solute should not be involved. Whether more fluid will vaporize depends only on the already existing partial pressure of vapor that has formed. –  Brian Feb 11 '13 at 19:13
    
@Brian: Actually all we were given was what I mentioned above, minus the formula. –  Yep Feb 11 '13 at 19:23
    
Hmm, I don't know what I was thinking, obviously it's the vapor pressure of the solvent (water) that's in question. Then it is very much dependent of the mole fraction of the solute. Sorry about that –  Brian Feb 11 '13 at 19:47
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1 Answer

up vote 1 down vote accepted

To solve this problem, use Raoult's law. In Raoult's law, the vapor pressure of a fluid is lowered proportionally with the mole fraction of solute particles in the fluid.

You know the formula, but you only need it if you want to calculate the ACTUAL resulting vapor pressure. We can know which is bigger like this:

A. 0.5M sugar. Sugar does not ionize, thus there is 0.5 M particles in the solvent.

B. 0.5M NaCl. Sodium chloride dissociates to two ions. Thus there is 0.5*2 M particles in the solvent.

C. 2M ethylene glycol. Again, this doesn't dissociate, so there is 2M particles.

D. 2M LiBr. Dissociates, resulting in 2 * 2 M particles.

Thus, listing the concentration of particles in the solvent, you see where the answer comes from:

D: 4 M

C: 2 M

B: 1 M

A: 0.5 M

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Makes perfect sense, thank you for the clear explanation! –  Yep Feb 11 '13 at 20:23
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