The more chemical components you add to a solution, the lower the vapor pressure becomes. Think of boiling point elevation - when you add salt to water, you increase boiling point. Vapor pressure works inversely. Something with a high boiling point has a low vapor pressure. The equation is just based on the amount of chemical component added in terms of the mole fraction, times the vapor pressure of the pure liquid to yield the new vapor pressure.
so P= X * P1
where P = pressure of the solution,
x = mole fraction
and P1 = vapor pressure of the pure solvent.
Hope that helps let me know if you have more questions.
