Question

Whats a vapor pressure?

like i know the definition but i cant get what it really means?!!

Answer 1

Well, what is the vapour pressure of WATER at `100` `""^@C`...?

Explanation:

And you should pipe up that the vapour pressure is `1*atm` (why? because this is the normal boiling point, at which the vapour pressure of the liquid is equal to one atmosphere...)

At lower temperatures, water exerts a lesser vapour pressure. For instance at `298*K`, `P_"saturated vapour pressure"=23.8*mm*Hg`. The full table is here.

Now water has exceptionally strong intermolecular forces, and this is evident when we compare its vapour pressure to that of other molecular substances, such as ethanol or diethyl ether, or hexanes....

And so the `"vapour pressure"` is the pressure exerted by the gas molecules that exist in equilibrium with the condensed phase at a given temperature, it is an excellent indicator of intermolecular force.

See here for another example.