Lab 8 Worksheet

Download the .pdf file of the lab handout here.

Experimental Procedure and Data

Diagram showing the hot plate towards the right, with the ring stand on the left (with the L pointing away from the hot plate). The ring stand holds the thermometer into the flask over the hotplate.
  1. Set up the ring stand, ring, tripod and hot plate as shown in the diagram.
  2. Clamp a thermometer to the ring stand to measure the temperature in the middle of the solution.
  3. Fill a 600 mL beaker with water and add a few boiling stones.
  4. Begin heating water.
  5. Obtain a clean, dry 125 mL Erlenmeyer flask
  6. Obtain a piece of aluminum foil and rubber band to serve as a cover for the experiment (but do not attach yet.
  7. Obtain the mass of the flask, foil and rubber band.
  8. Measure 4 mL of the unknown liquid.
  9. Pour the liquid into the 125 mL flask
  10. Fashion the foil tightly over the top of the flask with the rubber band.
  11. Punch a several small holes in the foil using a needle or paper clip.
  12. Immerse the Erlenmeyer in the water. The water level should be high enough to cover most of the flask but not so high as to allow water to enter through around neck near the foil.
  13. Clamp the flask using a second clamp to prevent the flask from bobbing up and down.
  14. Heat the water to about 85°C. You do not want the water to boil as that could cause splashing onto the top of the foil. Maintain this temperature (+/- 10 degrees).
  15. When the liquid in the flask has evaporated, you can move to the next step. (Usually this takes ~ 2-5 minutes but may take longer if the water bath was cool when you started.
  16. Remove the flask from the water and allow the flask to cool. Turn the heat source off.
  17. Wipe the outside of the flask to completely remove water being especially careful about drying the top edges.
  18. While the flask is cooling, obtain the pressure of the laboratory from the barometer at the front of the room.
  19. Only once the flask has cooled, obtain the mass of the flask set-up and record your data.
  20. Repeat this process (steps 5-19) for at least two trials. Determine the molar mass from separate trials.
  21. After the final trial remove the cover assembly and rinse the flask thoroughly.
  22. Use a graduated cylinder to fill the flask with water. Calculate the amount of water needed to fill the flask completely to the top (to the same volume the gas occupied). This may take several graduated cylinders of water. Be sure to add the volumes to the correct number of significant figures.

Pre-Lab Questions

*Note – This pre-lab must be completed before you come to lab.

1. Write the ideal gas law. Give the units for each variable.

2. A laboratory group measures an excess of an unknown liquid. They add this to an empty flask for which they calculate the volume. They heat the sample n a water bath until all liquid has been vaporized for several minutes. The sample is cooled and measured. Use their laboratory data (listed below) to answer the following questions:

Flask Volume 100.0 + 100.0 + 75.6 mL = 275.6 m L     .
Mass of flask 145.81 g
Mass of Flask + volatile liquid 149.25 g
Mass of volatile liquid 3.44 g
Temperature of water bath (°C) 84.1°C
Pressure in lab (atm) 0.914 atm
Gas Constant 0.0821 (L*atm) /(mol*K)

a. Calculate the mol of gas in the flask using the ideal gas law:

b. Calculate the molar mass of the unknown:

3. If a student added more than 4 mL of volatile liquid, would it affect the laboratory today? How? What if less liquid was added?

Results, Discussions and Post–Lab Questions

Unknown ID______________

Measurement Trial 1 Trial 2
Mass of empty flask, foil and wire
Mass of flask, foil, wire, and volatile liquid AFTER heating
Mass of volatile liquid
Pressure of Room (atm)
Temperature of water bath (°C)
Temperature of water bath (K)
Volume of flask (if more than one graduated cylinder of water added, add the volumes to obtain the final)
Mol of gas calculated from ideal gas law
Molar mass of gas

Average Molar Mass of unknown ___________________

Post–Lab Questions

1. Based on the molar mass you calculated for your unknown, what possible unknown (listed in Table 1) did you have? Calculate the percent error in the molar mass of your unknown based on this substance.

2. What were some sources of error in your experiment and how did they affect your measurements? (For full credit list at least 2 sources of error as well as how each affected the molar mass value you calculated.)

3. What would have been a more accurate way to perform today’s experiment?