Briefly explain why it was not necessary for you to determine the mass of unknow

Question

Briefly explain why it was not necessary for you to determine the mass of unknown liquid that you transferred to the flask before heating. Would the molar mass of a volatile liquid, calculated using the procedure in this experiment, Be incorrectly high, incorrectly low, or unaffected by the following procedural changes? (a) You did not completely vaporize the liquid when you heated it. (b) The foil cap got wet while you were cooling the flask and its contents with running water. (c) You added the boiling stone to the you had already determined the mass of the empty flask and foil cap.

Explanation / Answer

(1)

This is because we just want enough liquid that will fully vaporise and expel the air from the flask. Taking more amount of volatile liquid will not give any different result because ultimately the final amount of vapor remaining in the flask will be the same.

(2)

(a)

This will give an incorrectly high MW, since the recorded mass difference reading will be higher because of the presence of unvaporised liquid left in the beaker.

(b)

This will give incorrectly low reading, because some portion of the condensed liquid that should have been collected inside the beaker, has instead gone with the foil. So the mass difference reading will be low due to this loss.

(c)

The molar mass would be unaffected since the in the end the boiling stone will be removed while recording the reading for the mass difference.

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