Question

Dumas Method Experiment:

In this experiment, we measured a flask with an aluminum foil and elastic band set up then added 4mL of methanol inside the flask and sealed with the foil covering and boiled in a water bath until all the liquid vaporized (this will drive out any air in the flask and push out all excess vapour). Then we cooled it to let the vapour condense and reweighed the flask with the vapour in it. The following questions pertain to the experiment:

For each of the following experimental conditions determine whether the calculated value for molar mass would be: A) too high B) Too low C) Unaffected. In each case, explain how this result occurs.

a) After removing he flask from the water bath, the experimenter cools the flask to room temperature but does not dry it. The calculated molar mass will be: A) too high B) Too low C) Unaffected. Explain.

b) The flask is removed from the water bath containing vapour only, the experimenter cools the flask to room temperature and some vapour condenses inside the flask. The calculated molar mass will be:  A) too high B) Too low C) Unaffected. Explain.

c) The flask volume is not measured; instead the experimenter assumes the flask volume to be exactly 125.0 mL. The calculated molar mass will be:  A) too high B) Too low C) Unaffected. Explain.

d) From the time the mass of the unused flask assembly (flask, foil and elastic band) is recorded the flask is handled several times with oily fingers. The calculated molar mass will be A) too high B) Too low C) Unaffected. Explain.

Answer 1

For each of these cases, I'll answer shortly with a brief explanation so you can understand better.

A) The fact that the experimenter does not dry the flask, means that the minimun particles of waters are still in the flask and the molecular mass will be affected. In this case, it will be higher than usual, so, it will be too high.

B) Will be too low basically because the volume of the content is changing, and it's elevating, so this means that the mass would be go lower. The molecular weight will be low.

C) It will be unaffected because you are assuming a standard volume of the flask, so the molecular mass will be unaffected, there will be only a little range of mistake, but the average of the molecular mass will be the same.

D) In this case, it will be too high because oily fingers provide some mass to the flask, and it will be higher. So, unleast the experimenter dries the oil, the molecular mass will be higher.

Hoe this helps.