molecular mass = grams of vapor / moles of vapor Determine the average molecular
ID: 927222 • Letter: M
Question
molecular mass = grams of vapor / moles of vapor Determine the average molecular mass of the unkown from your three trials. (You may use the best two train if one is different from the others.) Questions: If the outside of your flask is not dry when the first mass determination is made and it is dried for the second mass determination, what effect will that have on your results? What will be the effect on your results if all of the liquid is not vaporized while heating? Which assumption would lead to the greatest error in your results, explain why? (a) assume temperature is 25 degree C, (b) assume the flask volume is exactly 125 mL, (d) assume the barometric pressure is exactly 760 torr.Explanation / Answer
1. If outside the flask is not dry then mass determination will include the mass of water(or other substances the flask is wet from). Mass of the vapor you get would be higher than the second time( when you completely dry the flask) . So the molecular mass derived from the first result would be higher.
2. There will be less volatile liquid left than there was initially; that is what we expect. You put in more than enough liquid so its vapor would fill the flask and the excess left through the pinhole. The condensed vapor (i.e., the remaining liquid) has the same mass as the vapor which exactly filled the flask. So if all the liquid is not vaporized the excess liquid we want to remove (in the form of excess vapor) will remain in the flask. As a result the mass of the vapor obtained will be higher which will lead to error.
3. If the flask you use is of 125 mL, then assuming it to be exacly 125 mL will lead to a considerable error in the determination of the volume. 125 mL is not an exact measurement; it refers to the volume of liquid it holds in normal use, so it excludes the volume of the flask's neck. This is why it is necessary to determine the volume of the flask by filling it with water (to overflowing).
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