Mass of empty stoppered flask 154.685 mass of flask, stopper and CO2 154.795 sto
ID: 489157 • Letter: M
Question
Mass of empty stoppered flask 154.685
mass of flask, stopper and CO2 154.795
stoppered flask volume 271
temperature 295.15 K
atmospheric pressure 772
Post Lab Questions:
Why was excess dry ice used in step 3 of the procedure?
Why do gas laws use degrees Kelvin rather than degrees Celsius?
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?
If you didn’t wipe away any frost or condensation formed during sublimation, how would your molar mass value be affected? Be specific please.
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.
Explanation / Answer
1) Excess dry ice was used to ensure that significant loss of CO2 doesn’t occur due to vaporization. The experiment requires us to loosely stopper the flask when dry ice starts to sublime and the flask is firmly stoppered when the entire dry ice has melted. However, some CO2 vapours will escape to the atmosphere. To ensure that significant error is not introduced in the calculations, an excess of dry ice is used.
2) Gas laws always used absolute temperature (or temperature in the Kelvin scale). Suppose a calculation of pressure and volume with the ideal gas law leads us to the situation
P1*V1/T1 = P2*V2/T2
Now, if one of the temperatures is 0C, then the calculation will lead to nothing since division by zero is not defined. However, when we use the temperature in the Kelvin scale, we are still working with the 273 K and the calculation will not be redundant.
3) The experiment will yield almost correct mass of CO2 since we are using the ideal gas law (P*V = n*R*T) to generate the value of n and hence the molar mass. The ideal gas law works fairly well for calculations at room temperature. This is due to the fact that gas molecules show little compressibility effects and hence their behavior can be approximated by ideal gas laws.
4) The recorded molar mass will be higher than the actual value. This is because the weight of the gas inside the flask will be higher (due to the condensate or the moisture). Since, we calculate the moles of the gas present using the ideal gas law and we define the molar mass of the gas as
Molar mass of the gas = weight of the gas/number of moles,
therefore, a higher mass of the gas (when number of moles are kept constant) will lead to a higher molar mass (ans).