Suppose that there are two chemical species in a sealed container(1L). X(l) is in phase equilibrium with X(g). The another one is Helium gas, which is 2 atm. If the vapor pressure of X at the given temperature is 0.5atm, what is the partial pressure of X in the sealed container? Should I take helium into account while calculating the pressure of X?

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    $\begingroup$ Your problem is not clear. The partial pressure of helium is 2 atm, and the partial pressure of X is 0.5 atm. Does it mean that the total gas pressure is 2.0 atm. Or is it 2.5 atm ? $\endgroup$
    – Maurice
    Mar 20 at 10:27
  • $\begingroup$ Actually, I was asking the same question as you. And I think the answer is 2.5atm, since the partial pressure of X is 0.5atm. $\endgroup$ Mar 20 at 12:49

1 Answer 1


Vapor pressure of X = the partial pressure of X at equilibrium conditions.

The total pressure has to be considered only in context as it very slightly increases vapor pressure of X by slightly increasing the chemical potential of the liquid. (You need more energy to add some volume of liquid to a system with higher pressure.)

  • $\begingroup$ Thank you for your kind answer! Now I understand that vapor pressure is the partial pressure that the volatile one has. Did I understand well? Also, Can I have some more explanation about chemical potential increase? I don’t understand it very well. $\endgroup$ Mar 20 at 12:52

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