Name(s): _____________________________________________________ Date: _________
The Molar Mass of a Volatile Liquid Lab
Introduction (From Vernier’s Lab Manual)
:
One of the properties that helps characterize a substance is its molar mass. If the substance in question is a volatile liquid, a common method to determine its molar mass is to use the ideal gas law, PV = nRT.
Because the liquid is volatile, it can easily be converted to a gas; while the substance is in the gas phase, you can measure its volume, pressure, and temperature.
You can then use the ideal gas law to calculate the number of moles of the substance. Finally you can use the number of moles of the gas to calculate the molar mass.
Safety Precautions :
Volatile liquids used in the lab are flammable liquids and fire risks. Wear chemical splash goggles, chemical resistant gloves, and a chemical resistant apron. Exercise care when working with the hot water bath. Wash hands thoroughly with soap and water before leaving the laboratory.
Material :
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LabQuest Pro
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2 utility clamps
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Unknown volatile liquid
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Balance
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Temperature Probe
Aluminum foil
Test tube
2 - 400 mL beaker
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Ring stand
Ice
Hot plate
Procedure:
1.
Trim a piece of aluminum foil so that it just covers the top of a small test tube. Use a needle to make a small hole in the foil.
2.
Using a balance, record the mass of the test tube and the aluminum foil
3.
Set up the apparatus required for this lab according to your teacher’s instruction using the ring stand and clamps.
4.
Prepare a boiling bath by heating about 300 ml of tap water in a 400 ml beaker. Keep the beaker on the hot plate once it is boiling
5.
Prepare an ice bath by adding ice to about 300 ml of tap water in a 400 ml beaker.
6.
Obtain about 0.5ml of the volatile liquid and pour into the previously massed test tube.
Cover the test tube with the aluminum foil quickly.
7.
Place the test tube in the boiling bath making sure that the aluminum foil is above the water line in the bath
8.
Immerse the temperature probe that is connected to a Vernier LabQuest system into the boiling water bath. Make sure that the probe is not touching the sides of the beaker or test tube.

9.
Keep the test tube in the boiling bath until it has complete vaporized. Some of the vaporized gas will escape through the small hole in the foil. This process serves to flush out the gases previously in the test tube. Keep the test tube in the boiling bath for at least
3 minutes after it has completely vaporized.
10.
Record the temperature of the boiling water
11.
Use a test tube holder to quickly transfer the test tube to the ice water bath. Cool the test tube for about one minute, then remove it and dry it completely.
12.
Record the mass of the dried test tube with the condensed volatile liquid and aluminum foil.
13.
Rinse out the test tube and fill it to the top with tap water and dry the outside. Cover the test tube with the aluminum foil and record the mass of the test tube, water, and foil.
14.
Repeat for a second trial
Data Table:
Trial 1 Trial 2
Mass of the test tube and foil cover
Temperature of the water bath
Mass of the test tube and foil and gas sample
Barometric pressure
Mass of test tube and foil and water

Data Analysis :
1.
Determine the mass of the condensed portion of the unknown that you placed in the test tube
2.
Use the mass of the water in the test tube and the density of water (1.0 g/ml) to calculate the volume of the test tube
3.
Use your data to calculate the molar mass of your unknown compound
4.
Find out the identity of the unknown substance from your teacher and calculate your percent error for its molar mass
5.
State how the following situations would affect the molar mass. Be specific (I.E. “The molar mass would be too large because…” a.
Only part of the test tube was immersed in the boiling water, so the temperature in part of the test tube was less than that of the water bath b.
The mass of the condensed liquid was not determined quickly. Instead, the test tube was allowed to stand for a while before immersing it in the ice bath and obtaining its mass c.
The vapor in the test tube did not condense to a liquid before you took the mass of the test tube