1. Briefly explain why it was not necessary for you to determine the mass of unk
ID: 1068977 • Letter: 1
Question
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.
(d) You forgot to measure the volume of the flask, so you used the volume printed on the flask for your calculations.
(e) Your unknown liquid had a boiling point of 102.3 Celsius.
3. The density of air at 100.0 deg. Celcius is less than 1.00g L^-1. Is it reasonable to assume that the vaporized liquid will force all of the air from the flask during heating? Briefly explain.
Explanation / Answer
1. Mass of unknown liquid is not needed prior to its transfer into the flask. The moles of gas formed from the liquid is calculated instead and the volume of flask is needed for calculating molar mass of unknown.
2. Errors
(a) If not all the liquid in the flask was vaporized, moles of gas would be lower, so the molar mass calculated would be incorrectly higher than actual value, molar mass = grams/moles
(b) If the foil cap got wet, the final weigth would be higher and thus moles would be higher which would give an incorrectly lower molar mass value.
(c) If boiling stone is added after weight measurements were done, the molar mass would remain unaffected.
(d) If incorrect volume of flask was taken for consideration, lower moles would be obtained and hence incorrectly higher molar mass value would be seen.
(e) If unknown liquid has boiling point higher than water boiling point, it will not vaporize at boiling water temperature. Incorrectly low molar mass seen.
3. Given the density of air, it is reasonable to assume that the vaporized liquid will force all the air our of the flask. The liquid vapor would occupy larger volume at higher temperature and would thus be able to rise and force the air out.
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