This is the equation that tells you what vapour pressure a liquid has at a certain temperature. It involves the enthalpy of vaporization, ΔH(vap) and a random constant that you'll probably never use (we call it "A"):
The reason you'll "probably never use it" is that most questions give you the ΔH(vap), and the vapour pressure at a certain temperature. Then, they'll ask you to predict the vapour pressure at a second temperature. To do this, you need to have a formula that relates two pressure and two temperatures. Here it is:
Given the enthalpy of vaporization is 41 kJ/mol and the Vapour Pressure at 373 K is 101.3 kPa, what is the vapour pressure at 298? Find out in the video below.
Maybe you'll care how the second equation comes from the first.
1. Divide one Clausius-Clapeyron Equation by a second one, as shown in the first step below.
2. Combine the exponents (remember exponent laws?)
3. Take the "ln" of both sides to "undo" the "e"
4. Factor out ΔH(vap) / R
5. Re-order the last two terms to clean up the expression.