11: Molar Mass of a Volatile Liquid
- Page ID
- 509309
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This lab was adapted from:
Kaya, J. J., & Campbell, J. A. (1967). Molecular weights from Dumas bulb experiments.
Journal of Chemical Education, 44(7), 394-395. https://doi.org/10.1021/ed044p394
- Students will...
- Students will...
INTRODUCTION
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 involves using 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 rearrange the ideal gas law to calculate the molar mass of the substance. (See Pre-Lab Assignment.) Also the gas constant that you will use in today’s lab is R = 0.082058 
.
PROCEDURE
Obtain a round-bottom flask, a stopper, a chimney adaptor, a 600 mL beaker and a vial containing an unknown volatile liquid. Carefully measure the mass of the empty, dry round bottom flask and stopper. Prepare a boiling hot-water bath by heating about 575 mL of tap water in a 800 mL beaker (875 mL in a 1 Liter beaker) using a hot plate. Also add 7-10 boiling chips. Obtain a LabQuest and connect a temperature probe (units °C). Make sure the mode is time based and set for 180 seconds (3 minutes).
Pour all of your unknown volatile liquid into the round bottom flask and place the chimney adaptor on top. Suspend the round bottom flask using a clamp and stand as demonstrated by your instructor. When the water is fully boiling, carefully lower the round bottom flask into the boiling bath as far as possible. Top off any extra space in the beaker with hot tap water.
Maintain boiling as your sample of liquid vaporizes. Note that some of your sample will escape the round bottom flask through the chimney. This process also serves to flush the air out of flask. Monitor the liquid level in the flask as it evaporates. Careful observation of the chimney should reveal a plume of gas escaping. While the liquid is evaporating. record the atmospheric pressure in the lab using the barometer. Then prepare a cold water bath. In a 400 mL beaker, add cold tap water and ice to make 375 mL total This will be used to cool the flask in a later step.
Once all the liquid is vaporized, hold the temperature probe in the boiling water being careful not to touch the sides or bottom of the beaker or the round bottom flask and begin data collection on the LabQuest. Keep boiling the round bottom flask for three minutes after all of the liquid in the round bottom flask has vaporized. When the 180 seconds ends, replace the chimney with the stopper. Then lift the round bottom flask out of the boiling water bath using the clamp as a handle. Place the flask in the cold water bath prepared above. (The clamp may be removed from the flask when convenient.) Every fifteen seconds or so, it is important to partially lift the stopper for a second to allow air to flow into the flask. Continue this process as you cool the flask for a total of 3 minutes.
Examine your temperature graph and record the highest temperature of the boiling-water bath, which will be used in the ideal gas law calculations. Now remove the flask from the cool water and carefully dry it completely. Partially remove the stopper one last time to equalize the pressure then measure the mass of the flask, stopper and condensed unknown liquid.
Determining total volume of flask with stopper inserted
Pour condensed liquid from your round bottom flask into the organic waste container in the hood. Rinse the round bottom flask with a few mLs of acetone and then with some DI water. Now fill the flask to the top with DI water. Insert the stopper carefully squeezing out water and not allowing air bubbles to form. Dry the flask and stopper completely. It would be ideal to obtain the mass of the entire flask at this point. But our balances can only weigh 200 grams at a time. Therefore you need to weigh the water that the flask now contains in several steps to get the total mass of the water in the flask. So you need to determine a procedure that obtains the total mass of water in the round bottom flask. Then convert the resultant mass of water into milliliters to calculate the total volume of the flask. Download a local copy of the linked Excel spreadsheet to enter your data. Return the round bottom flask, stopper, chimney and 600 mL beaker to the cart. Answer at least the first 3 questions on the next page before leaving lab today.
1. What was your procedure for determining the mass of water that filled the flask, and how did you use that to determine the volume of the flask? Include all data obtained and calculations needed for this determination.
2. Your unknown liquid is one of the following: hexane (C6H14), isopropanol (C3H8O), 2-butanone (C4H8O), ethanol (C2H6O), or 1,1,1-trichloroethane (C2H3Cl3).
Identity of the unknown liquid _________________
Show how you identified your liquid.
3. After identifying your unknown, open the linked Excel spreadsheet, switch the sheet to “Class Data” and enter your data.
4. If all the vapor in your flask had not condensed to a liquid when you cooled the flask, how would your calculations have been affected?
5. How would your experiment have been affected if you had used a different initial amount of the unknown compound?
6. Why does the stopper have to be partially removed so frequently as the flask cools?
7. Once there are at least 6 data points entered into the spreadsheet, copy the data into your local copy.
8. On your local copy, click in cell C3 (literature molar mass, first entry). Click the “Data” menu, then ”sort”. Choose “Sort by” then select “Literature” and click “OK”.
9. Now all the data for the same unknowns is together. Find the average % error for each unknown and the standard deviation of the % error for any unknown with three or more entries. Make sure these values are calculated in Excel and clearly labeled.
10. Compare the accuracy of the different unknowns. Which molecule was most accurate? Least accurate? Did the unknowns dominated by hydrogen bonding vs. dipoles vs London forces seem to exhibit any difference in accuracy?