Question

Question #f7a41

Answer 1

The temperature dependence of vapor pressure, or any other equilibrium constant, depends on the `Delta H` for that process, but we also need a reference point, which is the normal boiling point of water.

Explanation:

The ratio of vapor pressures at two different temperatures, `T_1` and `T_2` can be calculated approximately from the relation

`ln((P_2)/(P_1)) = (DeltaH_(vap))/R (1/(T_1)-1/(T_2))`

Using `T_1 = 373.15 K` (100 C), `P_1 = 1` atm,
and `T_2 = 310.15 K` (37 C)

`ln((P_2)/1) = (40700 J/(mol))/(8.314 J/(mol-K)) (1/(373.15 K)-1/(310.15 K)) = -2.66`

`P_2 = e^(-2.66)` atm = 0.0696 atm = 53.2 torr

The first equation ignores the (slight) temperature dependence of `DeltaH_(vap)`, but the true vapor pressure is only slightly less: 47.1 torr.