Question

Calculate the vapor pressure of acetone and n-hexane at their respective boiling points, 56.1degrees C and 68.7degree C and then calculate the percent deviation from the expected value. Recall that the normal boiling point is the temp at which vapor pressure=1atm
Hvap of acetone at 56.1degree C= 30.3kJ/mol
Hvap of n-hexane at 68.7degree C =28.9kJ/mol
andy help would be great. thanks!

Answer 1

You need the Clausius-Clapeyron equation.

Answer 2

No wait a minute...I read that too hastily. I don't actually understand the problem. At the normal boiling point the vapor pressure of either substance is 1 atm, by definition. So all I can think of is that you are being asked to calculate it theoretically using some model, and compare that result to 1 atm (the experimental result).

You could be meant to use the CC equation, if you're given the vapor pressure at some other temperature. It could also be that you're being asked to use the relationship between free energy change dG and the heat of vaporization dH and entropy change on boiling dS:\[\Delta G = \Delta H - T \Delta S\]Since dG = 0 at the boiling point, you can rearrange this to:\[T = \frac{\Delta H}{\Delta S}\] You're given dH, and you can estimate dS from the difference in standard molar entropies of the liquid and gaseous form of each substance, or even more crudely from Trouton's Rule.