# Chemical Forums

## Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Ice-cream on May 29, 2005, 10:44:27 PM

Title: vapour pressure of solution
Post by: Ice-cream on May 29, 2005, 10:44:27 PM
can any1 help me with this?

"Which of the following will have the lowest total vapour pressure at 25 degrees C?
a) pure water (vapour pressure = 23.8 torr at 25 degrees C)
b) a solution of glucose in water with X(C6H12O6) = 0.01)
c) a solution of sodium chloride in water with X(NaCl) = 0.01)
d) a solution of methanol in water with X(CH3OH) = 0.2 (Consider the vapour pressure of both methanol [143 torr at 25 degrees C] and water)"

I'm not sure whether it would be right if, say for b), I go P(total) = 0.99 x 23.8 = 23.562 (coz P of water would be 1 - 0.01 = 23.8)...i don't know becuase if that's right, then I will get the same answer for both b) and c)...but the answer at the back of the textbook says solution C will have the lowest total vapour pressure.
so can any1 tell me what's going on?!?!
Title: Re:vapour pressure of solution
Post by: ksr985 on May 30, 2005, 04:06:44 PM
NaCl happens to be a strong electrolyte. It will dissociate in water, giving Na+ and Cl-, and the actual mole fraction of solute will be 0.2. As a result, the lowering of vapour pressure will be greater in this case, and the vapour pressure itself will be lesser than in (b).