Vapor Pressure
A substance that is a liquid or a solid at room temperature is referred to as a vapor when it is
converted to the gaseous state. Therefore, the vapor pressure of a substance is the pressure produced
when equilibrium exists between its gas and liquid phase. In this laboratory investigation, you will
determine if different liquids have different vapor pressures and if the vapor pressure of a liquid can be
changed by heating or cooling the liquid. You will also determine if there is a cumulative effect of mixing
two liquids together.
Objectives:
Materials:
Compare the vapor pressure of different liquids.
Observe the effect of temperature on vapor pressure.
Observe the effect on vapor pressure when liquids are mixed.
Apparatus
Reagents
 Meter stick
 Distilled or deionized water
 80 cm of glass tubing
 Ethyl alcohol
 Clear vinyl tubing
 Methyl alcohol
 4 250-mL Florence flasks
 Acetone
 One-hole rubber stopper
 Mixed solvent solution
 10-cm glass tubing
 Ring stand and clamp
Procedure:
CAUTION: Ethyl Alcohol, methyl alcohol, and acetone are flammable. Make sure there are no
flames anywhere in the laboratory. Methyl alcohol is poisonous. Do not swallow any.
1. Put on your lab apron and safety goggles.
2. Check the apparatus to be sure that there is a volume of water in the tubing. Be careful to keep
the stopper level high enough so the water does not drain out.
3. To a dry Florence flask labeled “A”, add about 2 mL of acetone (CH3COCH3). Being careful not to
drain water into the flask, attach the pressure measuring apparatus. In Table 1, record the water
level in millimeters.
4. Without lifting the Florence flask from the lab table, gently cup the bottom with both hands for
30 seconds. Observe the effect this gentle warming has on the vapor pressure by measuring and
recording the new water level in Table 1. (The 30 second time can be increased or decreased to
maximize the effect or to minimize spillover. Just be sure that the warming time is consistent for
all 4 trials.)
5. To a dry flask labeled “M”, add 2 mL of methyl alcohol (CH3OH). ). Being careful not to drain
water into the flask, attach the pressure measuring apparatus. In Table 1, record the water level
in millimeters and repeat step #4.
6. To a third dry flask labeled “E”, add 2 mL of ethyl alcohol (CH3CH2OH). Being careful not to drain
water into the flask, attach the pressure measuring apparatus. In Table 1, record the water level
in millimeters.
7. Repeat step #4.
8. To a fourth dry flask labeled “A+E”, add 1 mL of acetone and 1 mL of ethyl alcohol. Being careful
not to drain water into the flask, attach the pressure measuring apparatus. In Table 1, record the
water level in millimeters.
9. Repeat step #4.
Data Presentation
Table 1. Vapor Pressure Data
Solvent
Water Level at Room
Temperature (mm
H2O)
Water Level after
Warming with Hands
(mm H2O)
Difference in Water
Levels (mm H2O)
Acetone
Methyl Alcohol
Ethyl Alcohol
Acetone and Ethyl Alcohol
Data Analysis
1. Prepare a computer generated vertical bar graph of each liquid tested, showing the difference in
heights of water. Be sure to have a title and label both axes with the label and the appropriate units.
Refer to Graphing Instructions on the web page for a refresher course.
Conclusions
1. How did warming the solvent affect the vapor pressure?
2. Based on the observations, compare the relative strengths of the intermolecular forces of attraction of
the three liquid samples. Tell which one is strongest, weakest and explain why.
3. How does the vapor pressure of a mixture of liquids compare to the vapor pressure of each of the pure
liquids? (A+E)