# How is mole fraction of a solution different in vapour state vs in solution vs vapour-liquid equilibrium?

'x' grams of benzene and 'y' grams of toluene, it is said that the vapor pressure of benzene is 75 mmHg and that of toluene is 22 mmHg. Find mole fraction in the vapor state,

What I do not understand is how different states can have different values for mole fraction? Do the moles change with a change of state?

More precisely how does the fraction of mole in each state look at transition states? As in say I started with 100 moles in the liquid phase, when it reaches the point where it is about to turn to gas (b.p) how much fraction of moles are gas and how much are liquid?

• HINT - Assume that the liquid solution is an ideal liquid mixture. – MaxW Nov 15 '19 at 19:26
• how does the answer the title question? – Buraian Nov 15 '19 at 19:31
• As benzene is more volatile than toluene, it should be clear to you the vapour composition must be enriched by benzene, compared with liquid composition. If ideal behaviour is implied , vapour composition is interpolation of vapour composition over pure liquids, if liquid molar fractions are considered. – Poutnik Nov 16 '19 at 7:20

## 1 Answer

When you have a liquid mixture, there is a vapor phase that is made up of those same molecules in equilibrium with the liquid phase. There is no rule that says the moles will split 50/50 in the vapor and liquid phase or any other fraction. The amount that stays in any one phase is influenced by many variables (e.g. molecule to molecule interactions, temperature, pressure, volatility) so we use many different models and assumptions to calculate the amount in either phase or we conduct experiments to measure the amounts.

• I can not grasp how the number of particles change in each state,where does all the other moles go to? – Buraian Nov 16 '19 at 4:28
• @DDD4C4U See Poutnik's comment for some further clarification. Can you leave a comment describing the mental picture you have about the situation? At a high level, you should consider how molecules are able to form the vapor phase - some molecules from the liquid have enough energy to escape the molecular interactions within the liquid. The number of those molecules that have that energy dictates the molecule split. The total number of molecules in the vapor and liquid phase, in a closed system, will be constant. – J. Ari Nov 16 '19 at 12:36