What is vapor pressure? The simple way to understand it, is to realize that pressure is caused by gas (in our case vapor). The more gas there is, the higher the pressure is. Vapor pressure is the pressure of the gas above the liquid. The more vapor there is above, the higher the vapor pressure is.
Intermolecular forces are attractive forces between molecules. The stronger the intermolecular forces, the more attracted the molecules in liquid are to each other and the harder it is to separate them to turn into gas (vapor). Therefore, for a liquid with strong intermolecular forces, not a lot of vapor will be formed and the vapor pressure will be low. A liquid with weak intermolecular forces, such as methane, CH4, will have high vapor pressure.
Conclusion, intermolecular forces and vapor pressure are inversely proportional. The higher the intermolecular forces the lower the vapor pressure and vice versa.
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