Post-Laboratory Questions Use the spuces procided for the ansvers and additional
ID: 1025001 • Letter: P
Question
Post-Laboratory Questions Use the spuces procided for the ansvers and additional paper if necessary) 1. Briefly explain why it was not necessary for you to determine the mass of unknown liquid that you transferred to the flask before heating 2. 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 flask after you had already determined the mass of the empty flask and foil cap.Explanation / Answer
1-
The molar mass is calculated using:
Molar mass = mass/moles of vapor
where mass = mass of sealed flask with vapor - mass of empty flask
= mass of trapped vapor
It is the mass of the vapor trapped in the flask that is used in the calculation and not the mass of the unknown liquid that is transferred into the flask
=> not necessary to determine mass of unknown liquid.
And The mass of the liquid before heating, thus differs by much to that of actual mass of liquid. Sometimes, the compound itself is volatile enough that gets evaporated during the reaction processes and may alter the calculations by much. By heating, the volatile part is burned first and then the analysis is carried out in order to avoid the possibility of errors.
So, there's no significance of taking the mass of unknown liquid without heating. You might end up taking the mass of impurities along or you would end up getting incorrect experiment due to the calculation error caused by volatility of liquid.
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