Background-Overview: The ideal gas law relates the four measureable properties of a gas: pressure, volume,
number of moles and temperature. In this experiment, the ideal gas law will be used to determine the molar
mass of gases and volatile liquids. A small sample of the volatile liquid is placed in a tared 15 mL plastic pipet
and the pipet is heated in boiling water to vaporize the liquid. The air and excess vapor escape, leaving the
pipet filled only with the volatile liquid vapor at atmospheric pressure and at the temperature of boiling water.
The pipet is then removed and cooled to condense the vapor. Once cooled, the pipet is massed. By massing the
same pipet filled with deionized water, the volume of the pipet is calculated. The molar mass of the volatile
liquid is then determined using the mass of the condensed vapor, the volume of the pipet, the atmospheric
pressure and the temperature of the boiling water.
Ideal Gas Law:
PV = nRT
P= Barometric pressure (in atm)
R= 0.8206 L · atm/mol · K
V=Volume (in L)
T= temperature (in Kelvins)
n= number of moles
Molar Mass:
Molar mass = mass of condensed vapor/moles of condensed vapor
Purpose: The purpose of this experiment is to determine the molar masses of various volatile liquids. The
liquids are volatilized and condensed in a fixed volume. The condensed vapor is massed and the liquid's molar
mass is calculated from the experimental data.
Safety:
-The liquids being used are flammable and fire risks. Ethanol is made poisonous by the addition of denaturant-it
cannot be made nonpoisonous.
-Wear chemical splash goggles.
-Exercise care when working with the hot water bath
-Wash hands thoroughly with soap and water before leaving the laboratory
Materials:
-Chemicals: acetone, ethyl alcohol, isopropyl alcohol, deionized water
-Equipment: 0.001g balance, barometer, 250mL beaker, two 400mL beakers (one shared with another lab
group), three jumbo pipets, hot plate, permanent marker, test tube clamp, thermometer, plastic tubing pipet
holder, pliers, scissors.
Pre-Lab:
Carefully read the procedure. A determination of the molar mass of methanol yielded the following data:
Temperature of boiling water bath
99.5 °C
Barometric Pressure
738 mm Hg
Temperature of Room Temperature water bath
24.0 °C
Mass of empty pipet
1.557 g
Mass of pipet and condensed methyl alcohol
1.571 g
Mass of pipet and Water
16.001 g
1.
Determine the mass of the condensed methyl alcohol.
2. Determine the mass of water in the filled pipet.
3. Use the relationship of mass and density to determine the volume of the pipet (which is equal to the volume
of water in the pipet.
4. Using the Ideal Gas Law, calculate the number of moles of methyl alcohol.
5. Using the number of moles of methyl alcohol and the mass of the condensed methanol, determine the molar
mass of the methanol.
6. Compare this to the actual molar mass of methyl alcohol.
7.
Why is the vapor unlikely to behave as an ideal gas near the temperature at which the vapor would liquefy?
8. The following mistakes were made when carrying out the experiment. What effect does each have on the
calculated molar mass? Be specific. For example, too large because....
a. Only part of the pipet was immersed in the boiling water, so the temperature in part of the pipet was
less than the water bath.
b. The mass of the condensed liquid was not determined quickly. Instead, the pipet was allowed to stand
for a while before immersing it in room temperature water and then massing the pipet.
Procedure:
1.
Place a 400 mL beaker on the hot plate and add about 300mL of water to the beaker. Turn on the hot plate
to boil the water. When the water in the beaker begins to boil, adjust the temperature of the hot plate so
that the water remains boiling but does not splash from the beaker.
2. Obtain three jumbo Beral-type pipets. With pliers on the back
or front desk, pull the thin stems o each so that a fine "capillary"
tip is formed where the stem has been pulled.
3. Cut the pipet as shown in Figure One so that the capillary tip is
less than 1cm long.
4. Label the pipets #1, #2, and #3 with the permanent marker.
5. Mass each pipet to the nearest 0.001g and record this mass in
your log book data table.
6. Draw 2-3 mL of ethyl alcohol from the labeled bottle into each of the
previously prepared and labeled pipets.
7. Insert the tips of the filled pipets into the short piece of plastic
tubing and secure with the test tube clamp.
8. Immerse the pipets in the boiling water so that the entire bulb of each
pipet is below the water line.
9. Heat for at least five minutes.
10. Record the temperature of the boiling water bath and the barometric pressure of the room in the log book
data table.
11. Carefully remove the pipets from the water. Inspect each pipet. if any liquid remains in a pipet bulb, heat
the entire assembly for another minute.
12. Cool the pipets by lowering the pipet assembly into a bath of room temperature water in the second 400mL
beaker.
13. Dry the pipets with paper towels and mass each pipet which now contains only the condensed vapor, to the
nearest 0.001g. Record these values in your log book data table.
14. Fill a 250mL beaker with room temperature deionized (DI) water.
15. Fill pipet #1 with the DI water and then expel the water into the sink to flush the remaining ethyl alcohol
from the pipet. Repeat this process three times.
16. To determine the volume of pipet #1: Fill the pipet completely with DI water, dry the outside carefully, and
mass the pipet and water. Record this mass in your log book data table.
17. Repeat with pipets #2 and #3.
18. Empty the pipets of water.
19. Repeat steps 6 through 13 for acetone and isopropyl alcohol.
20. Clean your lab station.
See separate sheet for Data and Analysis and Conclusion Questions.
Name ________________________________________ Period _____ Date ______________________
Partner(s) _______________________________________________________________
Determination of the Molar Mass of Volatile Liquids
PRELAB
Answer the questions based on the table located in the pre-lab
1.
Determine the mass of the condensed methyl alcohol.
2. Determine the mass of water in the filled pipet.
3. Use the relationship of mass and density to determine the volume of the pipet (which is equal to the
volume of water in the pipet.
4. Using the Ideal Gas Law, calculate the number of moles of methyl alcohol.
5. Using the number of moles of methyl alcohol and the mass of the condensed methanol, determine the
molar mass of the methanol.
6. Compare this to the actual molar mass of methyl alcohol.
7. Why is the vapor unlikely to behave as an ideal gas near the temperature at which the vapor would
liquefy?
8. The following mistakes were made when carrying out the experiment. What effect does each have on
the calculated molar mass? Be specific. For example, too large because....
a.
Only part of the pipet was immersed in the boiling water, so the temperature in part of the pipet
was less than the water bath.
b. The mass of the condensed liquid was not determined quickly. Instead, the pipet was allowed to
stand for a while before immersing it in room temperature water and then massing the pipet.
Data and Analysis -attach the data table to this sheet:
1.
Create a data table with the following information: Temperature of boiling water bath (°C), Barometric
Pressure (mmHg), Temperature of room temperature water bath(°C), Density of water at room
temperature (g/mL), For all three jumbo pipets: Mass of empty pipet, Mass of pipet and water, Mass of
water in filled pipet, Volume of pipet.
2. Create a second data table with the following for each liquid tested: mass of pipet and condensed liquid,
mass of condensed liquid, molar mass of liquid. Be sure to list the name of each liquid with its
information.
Attach the data tables to this sheet.
Conclusion – attach conclusion questions to this sheet:
1.
Determine the mass of the condensed, volatile vapor for each pipet trial. Enter these values in the data
table.
2. Use the CRC Handbook of Chemistry and Physics to determine the density of water at the temperature
of the room temperature water bath used in this experiment. Enter this density value in the data table.
Use this value and the mass of water in each filled pipet to calculate the volume of each pipet.
3. Determine the mass of the condensed volatile liquid for each run. Enter these values on the data table.
4. Calculate the molar mass of the liquid used in each run and the average of the three runs for each
volatile liquid.
5. Volatile liquids with lower boiling points often give better results than those with higher boiling points.
Suggest a reason for this.
6. What effect would vapor condensation in the neck of the jumbo pipet have on the reported molar mass?
How large an error might this introduce?
7. Some liquids have enough attractions between molecules to form dimers. Dimers are molecules formed
from the combination of identical molecules (A + A →A2). What effect would this have on the
experimental molar mass?
8. Look up the true molar masses for your liquids. Calculate your percent error for each. Show your work.
Attach the conclusions after data tables to this sheet.