I have an experiment to measure the molar mass of CO2. Procedures: 1. Obtain a t

Question

I have an experiment to measure the molar mass of CO2.

Procedures:

1. Obtain a thermometer and place it on the bench at your work area. Be careful it does not roll off the bench.

2. Weigh an empty Erlenmeyer flask with a rubber stopper firmly capping it. Record the mass.

3. Place a small piece of solid CO2 (dry ice) in the flask (use about twice the amount you calculated in pre-lab question c). Place the stopper loosely on top of the flask. Watch the flask while the CO2 sublimes. Firmly stopper the flask immediately after all the solid CO2 disappears.

4. Wait until the flask is equilibrated to room temperature (around 5-10 minutes), and then “burp” it by lifting one side of the stopper, and then quickly pushing it back in. This is to ensure that the pressure is the same inside and outside the flask.

5. Dry off any accumulated moisture from the outside of the flask, weigh the stoppered flask (on the same scale) and record the mass.

6. To determine the volume of the flask: fill the flask all the way to the brim with water and stopper it while holding it over a sink. If there is any air trapped inside, try again until only water is present inside the stoppered flask. The volume of the water should be the same as the volume of the air in the stoppered flask. Dry off the outside of the flask completely. Measure the volume of water in a graduated cylinder in batches and add up the volumes to get the total volume of all the water in the flask. Record this as the stoppered flask volume.

7. Read the thermometer in your work area and record the temperature.

8. Read the barometer in the lab and record the pressure in mmHg. Ask me if you need help with the barometer. Don’t forget to correct the barometer reading for room temperature using the table hanging on the wall.

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And here are the questions:

1. Why was excess dry ice used in step 3 of the procedure?

2. Why do gas laws use degrees Kelvin rather than degrees Celsius?

3. Were you surprised at how accurately the mass of a molecule of CO2 could be determined with such simple equipment? Explain why or why not?

4. If you didn’t wipe away any frost or condensation formed during sublimation, how would your molar mass value be affected? Be specific please.

5. Why do we measure the volume of the flask with water rather than using the volume listed on the side?

6. Why is the flask not firmly stoppered during sublimation?

Explanation / Answer

1. so that all the other gases could escape away from the flask and only carbon dioxide is left inside (being heavier than oxygen and nitrogen).

2. because kelvin scale starts at absolute zero where the entropy is zero and any motion/randomness starts above this point.

3. no, because when we take bulk amount of any element and determine its weight with such accuracy which has to be distributed among moles of molecules then the chances of error decrease.

4. if the frost wasn't wiped away then the weight of water condensed on the flask would also come into the consideration of molar weight and we would have calculated an increased molecular weight of CO2.

5. Because the volume listed on the flask is never exactly the volume so to determine the volume of the falsk accurately water has to be used to measure the volume to the nearest 10th of a millilitre.

6. so that the whole flask is filled with carbon dioxide gas only for accurate determination of its molar mass otherwise the weight calculated would be the weight of the carbondioxide-air mixture.

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