A.P. Chemistry      Name:                                                      Date:

Laboratory                                                  Mr. Kimberly


Molar Mass of a Volatile Liquid




            In class, we have discussed how the molar mass of a gas can be determined from its density.  We will employ this relationship in today’s lab to determine the molar mass of an unknown volatile liquid.  While research chemists use sophisticated equipment to accurately measure the molar mass of an unknown substance, you will see that the simple method employed in this lab works well.




1.     Fill a 1000 mL beaker with ~525 mL water.  You can use the graduations on the beaker to measure the volume.

2.     Place the filled 1000 mL on a hot plate in the fume hood, and turn the heat dial to #5 setting.  You will use this beaker as a heating bath.

3.     Prepare a 500 mL Erlenmeyer flask by washing it with a drop of soap and water.  Use a paper towel to dry it completely.  You can use the handle of one of the scrub brushes to push the paper towel inside to dry it.

4.     Cut a square of aluminum foil and cover the top of the flask.  Trim the foil so it neatly covers the mouth of the flask, but does not extend far down the neck.

5.     Mass the dry, empty flask with the foil cover.  You will need to use the analytical balance (210-g capacity) in the prep room for this—the other analytical balances do not have a capacity large enough.

6.     Check the hot water bath to make sure it is not boiling over.

7.     Go over to the unknown liquid, and fill the disposable pipette with ~2 mL.  One partial depression of bulb should suck up this volume.  Transfer the unknown liquid to your flask.

8.     Re-cover the flask with the aluminum foil, and using a pin, punch a small hole in the top.

9.     Text Box:  If the heating bath is not yet boiling, you may want to turn the heat up a little until it begins to boil lightly.  (You will probably need to turn the heat back down later.)

10.  Use a ring stand and a single buret clamp to hold onto the neck of the flask, and immerse the flask as far as you can into the 1000 mL beaker.  The final set-up should look like the diagram on the right.

11.  Watch the liquid carefully.  When it appears that all the liquid has vaporized (you no longer observe liquid streaking down the sides), continue to heat for another minute.

12.  Remove the flask from the hot water bath, using the buret clamp.  (Unscrew it and slide it up the ring stand).  Return to your lab bench, set the flask down, and before you let go, remove the clamp from the neck of the flask.

13.  Allow the flask to cool to room temperature.  While this occurs, you should record the temperature of the boiling water and the atmospheric pressure.  You can read the barometric pressure at the weather station in the exhibition room on the first floor of Weaver Hall.

14.  When the flask has cooled completely, make sure the outside of the flask is dry, and then find the mass the flask and foil cover again.

15.  If time remains, repeat the procedure again from step #7. 

16.  If the second trial does not produce a result (mass of vapor) that is within 5% of your first trial, then, if there is time, perform the procedure again until your results agree.

17.  When you have performed all of your trials, fill the flask with water up to the top.  Pour all the contents into a 1000 mL graduated cylinder, and measure the volume to the ones place to give you three significant figures.




            This will be an informal lab write-up.  You may type it, or neatly write it up in your lab notebook.  Here are the things I would like you to include:



Create a table to present all your measurements (mass, volume, pressure, temperature) for each trial.


·      Use your measurements to calculate the molar mass of the vapor (volatile liquid) for each trial.

·      If you performed more than one trial, you need write out the calculations for only one of your trials.


1.     In chemical research, what is the most common (and the most accurate and expensive) means by which to determine the molar mass of an unknown substance?  Describe in a sentence or two how this method works.

2.     At the end of the experiment, why do you have to let the liquid condense before finding the mass of the flask?

3.     Given your calculations in the previous section, use the information below to determine the identity of the unknown liquid:

Substance Name

Chemical Formula

Molar Mass (g/mol)




ethyl acetate



ethyl alcohol (ethanol)



methyl alcohol (methanol)



Error Analysis

1.     Use the molar mass of the unknown liquid given in the table above to calculate the percent error for the experiment as a whole.

2.     What are two sources of error, and how would they impact your results?  Be specific in your answer.