TOPIC: Solutions

Demo-016

Vapor Pressure of a Solution

This demonstration should accompany an introduction to the properties of solutions. It is a good way to show qualitatively that the vapor pressure of a homogeneous solution is the weighted average of the vapor pressures of the components (Raoult’s law).

MATERIALS

Two mercury-filled open ended manometers with quick disconnects on hoses
Two filter flasks (500 mL) with rubber stoppers configured with rubber septa (see figure) and quick disconnects
Several 35 mL plastic hypodermic syringes with needles
Several 100 mL beakers
100 mL samples of methyl and ethyl alcohol and water

Figure. Apparatus for Demonstrating Solution Vapor Pressure

PRESENTATION

This demonstration should accompany your discussion of the properties of solutions. It can be used to introduce Raoult's law which guarantees that the equilibrium vapor pressure of a homogeneous solution of two substances is the weighted average of the vapor pressures of the pure components. This demonstration does not prove this, but merely shows that vapor pressures are not additive when two volatile liquids are mixed.

Begin by setting up two universal gas apparatuses. Inject one with methyl (or ethyl, depending on how warm the room is) alcohol and the other with water. The two vapor pressures will be significantly different. Ask your class what will happen to the observed pressure of an equal volume of water is injected into the flask already containing the methyl alcohol. Will the pressure go up? Why? Why not? Discuss the possibilities before doing the experiment. Inject water into the flask containing the methyl alcohol. The observed vapor pressure will decrease.

DISCUSSION

When two liquids are miscible, the vapor pressure of the homogeneous mixture is the weighted average of the vapor pressures of the components. The weighting factors are the fraction of each type of molecule present in the solution (mole fraction). This is known as Raoult's law (below).

Psolution = Xcomponent A ´ Pocomponent A + Xcomponent B ´ Pocomponent B + ...

In the equation the Xs are mole fractions and the Po quantities are the vapor pressures of the components when pure. It is obeyed accurately when the mixed liquids have similar intermolecular forces of attraction. This is common when the two liquids are chemically similar, e.g., a solution of pentane and hexane would obey Raoult's law to a high degree of accuracy

HAZARDS AND DISPOSAL

Don't stab yourself with the hypodermic needles!

 

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