Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

Plot of the logarithm of vapor pressureversus 1/T

The liquids shown in Figure 11.7 are replottedin this graph. Note the straight-linerelationship.

A two-point form of the Clausius–Clapeyron equation is very useful for calculations. Let us write the previous equation for two different temperatures:

Here P₁ is the vapor pressure at absolute temperature T1, and P2 is the vapor pressure at absolute temperature T₂. If you subtract the second equation from the first, you get

which you can write as

The next two examples illustrate the use of this two-point form of the Clausius–Clapeyron equation

Example 11.2 Calculating the Vapor Pressure at a Given Temperature Estimate the vapor pressure of water at 85°C. Note that the normal boiling point of water is 100°C and that its heat of vaporization is 40.7 kJ/mol.