vapour pressure in AP Chemistry

Questions involving vapour pressure appear each year in the AP chemistry exam. To understand the concept of vapour pressure, the following should be kept in mind:

Vapour pressure is the pressure exerted by the gas above a liquid in a closed container when equilibrium has been established.   When a liquid is introduced to a closed container, the molecules with higher kinetic energy at the surface of the liquid can escape and go to gas phase. At the same time, the molecules in the gas phase with lower kinetic energy could be trapped back into the liquid phase due to their intermolecular interactions with the liquid molecules at the surface. This is a dynamic process. When equilibrium is established, the number of molecules escaping to the gas phase equals to the number of molecules entering the liquid phase and the vapour pressure remains constant.

Vapour pressure is only dependent on the nature of the substance and temperature. 

The stronger the intermolecular forces in a certain substance, the lower the vapour pressure.

The higher the temperature, the higher the vapour pressure. At higher temperature, the molecules has higher kinetic energy, making it easier for them to escape and stay in the gas phase.

The temperature at which the vapour pressure reaches the atmosphere pressure is called the boiling point. when the atmosphere pressure is 1atm, the boiling point is called normal boiling point. At higher elevations, the atmosphere pressure is lower, thus the boiling points at higher elevations is lower than at sea level for a specific substance.

When two or more volatile substance are mixed together, the total vapour pressure is the sum of the product of each individual ingredients' mole fraction and vapour pressure.

When non volatile substances are dissolved in a solution, the solute molecules at the surface of the solution interfere with the vaporization process, resulting in lower vapour pressure, which in turn elevate the boiling point of the solution. The magnitude of boiling point elevation is proportional to the molality of the solution as well as the particle numbers of the solute when it dissociate in solution. It has been stated in the new curriculum framework that colligative properties is no longer covered by the AP chemistry exams. It does not hurt to know it though.

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