Question

A binary solution of two liquid A and B is formed.Liquid B is more volatile than liquid A.If external pressure over liquid mixture gradually decrease then-
A).Mole fraction of A in liquid phase increase.
B).Mole fraction of B in liquid phase increase
C).Mole fraction of A in liquid phase decrease
D).Pressure fraction of B increases

Answer 1

what do you think, and why?

Answer 2

C bcoz the pressure has been relieved so more no. of moles can easily go out of the solution..
Bt they have given something else

Answer 3

Well it is said that B is more volatile than A which means it has a higher vapour pressure leading to a low boiling point.
We can all say it in this way also. Imagine we increased the temperature by 40 centigrades. Then more from B should have gone to vapour phase compared to that of A because B has a low boiling point thus it goes to the vapour phase when a considerable amount of heat is given.
Here it is the external pressure divided by liquid mixture. For the value of this to decrease either the external pressure should decrease or the liquid mixture should increase

Answer 4

It is given that the solution is a binary solution which means no. of moles of A is equal to the no. of moles B because they haven't said anything about the change of moles, by addition or what so ever It's safe to go with the decrement of external pressure as the cause of decrement in the external pressure over liquid mixture. Because B is more volatile than A , more of B is present in the vapour phase, if the external pressure decreases that means B in the vapour has decreased. But B can't go any where else right? It should have gone to liquid phase thus increasing the mole fraction of B in liquid phase

Answer 5

This is what I think so then the answer would be B ! Am i right ? @Cuanchi

Answer 6

I think your logic is good @rushwr.
It is not clear in the problem what is the reason of the decrease of the external pressure. If the external pressure over liquid mixture gradually decrease because a change of the temperature of the system (in this case will apply your logic) and more B has to be dissolve in the liquid phase.
But if B is more volatile than A and the temperature didn't change and you have more B in the liquid phase the pressure will increase.
The other option is that the temperature doesn't change and the pressure decrease because increase the volume or removing the gas phase. In this other case the answer will be A).Mole fraction of A in liquid phase increase.
Besides, if you choose the answer B as a correct, then the answer C is also going to be correct.

I will go for A).Mole fraction of A in liquid phase increase.

Answer 7

What if we take the external pressure created is the vapour pressure of the system !

Answer 8

Actually we can't say that A in the liquid phase decreases right ? Because A is anyway less volatile than B. So if the B is in the liquid phase there's no way of A to go to the vapour phase

Answer 9

The amount of A in the liquid phase may not change but the problem is saying the molar fraction of A.

Answer 10

If the molar fraction of B increase then the molar fraction of A has to decrease

Answer 11

oh crap yes I was taking it as moles. I'm soo careless !!!!!! Dumb dumb dididi sry abwt that ! @Cuanchi but is there anyway that A can be true !?

Answer 12

Because it said is a solution of two liquids, I assume there is no chemical reaction between the liquids neither the gas. So then will be a moment that the gas and liquid will be in equilibrium. From that moment neither the pressure neither the composition of the two phases will change.
So I assumed that the pressure decrease is an external phenomenon, they are increasing the volume or removing gas phase by vacuum. Then more B move to the gas phase and decrease the molar fraction of B in the liquid (increase of molar fraction of A in the liquid)

Other possibility is that there is a chemical reaction in the gas phase and that is making the pressure decrease, in this case also more B will move from the liquid to the gas phase, decreasing the molar fraction of B.

Answer 13

What exactly do they man by liquid mixture

Answer 14

mean*

Answer 15

Okay! So @cuanchi what u want to convey is that if pressure somehow decrease over the system then more no of moles of B can go into the vapour phase,thereby decrease in mole fraction of B in liquid phase in binary mixture.
And if this happens then no. of moles of A increases in the solution since Xa=1-Xb
Xa ,Xb are mole fractions of A and B respectively..
Isn't it??

Answer 16

yep that was what he wanted to say i guess ?

Answer 17

And if this happens then no. of moles(molar fraction) of A increases in the solution since Xa=1-Xb
Xa ,Xb are mole fractions of A and B respectively..
Isn't it??

Answer 18

@cuanchi would you reply to this??

Answer 19

yea the only observation that I did was that you wrote "number of moles" of A increase, and is not the number of moles but the "molar ratio". I think that the concept of molar ratio is clear for you since you also wrote " Xa=1-Xb "
that's all

Answer 20

and we think that the correct answer is
A).Mole fraction of A in liquid phase increase.

Answer 21

Okay! Thanks..u made it more clear though!