a student ways an empty flask and stopper in find the mass to be 54.868g. she th

Question

a student ways an empty flask and stopper in find the mass to be 54.868g. she then adds about 5 ml of unknown liquid and heats the flask in a boiling water bath 100°C. after all the liquid is vaporized she removes the flask from the bath, stoppers it, and let's it cool. after it is cool she momentarily removes the stopper, than replaces it and weighs the flask and condensed vapor, obtaining a mass of 55.496g. the volume of the flask is known to be 235.7mL. the barometric pressure in the laboratory that is 738mmhg.

a. what was the pressure of the vapor in the flask in atm?

b. what was the temperature of the vapor in K?

c. what was the mass of vapor that was present in the flask

D. how many moles of vapor are present?

e. what is the mass of one mole of vapor?

Explanation / Answer

flask = 54.868g
5ml liquid at 100ºC
mass flask after heating = 55.496g
volume flask = 235.7ml
p = 738mm

a) p = 0.97atm

b) t in K = 373K, v = 0.2357L

c) mass gas = 55.496g - 54.868g = 0.628g

d) n = pv/rt = (0.97atm x 0.2357L) / (0.0821L-atm/mole-K x 373K) = 0.0075 moles
e) mass 1mole gas = 0.628g / 0.0075moles = 83.73g/mole

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