Question

At 87°C the vapor pressure of benzene is 931 torr and that of toluene is 290 torr. Which substance, benzene or toluene has the higher normal boiling point? Is the normal boiling point of benzene above or below 87°C? Is the normal boiling point of toluene above or below 87°C? Explain how you arrived at your answers.

Answer 1

First of all you have to know the definition of boilling point. So the boilling point is that temperature at which the vapour pressure of liquid is equal to the atmospheric pressure.

Now we know that the vapour pressure is inversely proportional with boilling point as let's think if the intermolecular forces are small then liquid has high vapour pressure so little heat will have to be added to seperate the molecules thus the boilling point is low.

So as at 87°c the vapour pressure of Benzene is higher than Toluene this the boilling point of Toluene will be higher than that of Benzene.

Now coming to the next para of the question, As we know atmospheric pressure is equal to 760 torr (1atm) so to boil any liquid or solution the vapour pressure must be equal to atmospheric pressure. Now at 87°c for Benzene the vapour pressure is 931 torr which is greater than the atmospheric pressure (760 torr) thus the boilling point of Benzene is below 87°c .

For Toluene at 87°c the vapour pressure is 290 torr which is lower than the atmospheric pressure (760 torr) so to boil Toluene more temperature will be needed. Thus the boilling point of Toluene is above 87°c .