Advance Study Assignment: Molar Mass of a Volatile Liquid 1. A student weighs an

Question

Advance Study Assignment: Molar Mass of a Volatile Liquid 1. A student weighs an empty nask and stopper and finds the mass to be 53.256 g. She then adds about 5 mL of an unknown liquid and heats the flask in a boiling water bath at 98.8 C. After all the liquid is vaporized, she removes the flask from the bath, stoppen it, and lets it cool. After it is cool, Nhe momen- tarily removes the stopper, then replaces it and weighs the flask and condensed vapor, obtaining a mass of 5378 og The volume of the flask is known to be 231.1 ml The absolute atmospheric pressure in the laboratory that day is 728 mm Hg. a What was the pressure of the vapor in the nask in atan? a b What was the temperature of the vapor in K?the volume of the flask in liters? K v L c. What was the mass of condensed vapor that was present in the flask? grams d. How many moles of condensed vapor were present? in moles e. What is the mass of one mole of vapor (Eq. 2? MM grmole (continued on following page)

Explanation / Answer

1. For the gas,

a. pressure of gas = 728/760 = 0.96 atm

b. Temperature = (98.8 + 273) = 371.8 K

Volume = 0.2311 L

c. mass of gas = 53.780 - 53.256 = 0.524 g

d. moles of gas = PV/RT

                         = 0.96 x 0.2311/0.08205 x 371.8 = 0.0073 mols

e. mass of one mole of gas = 0.524 g/0.0073 mol = 71.8 g/mol

2. Effect of experimental errors on results

a. If not all of the liquid was vaporized, the mass of vapor would be lower. This would reduce the molar mass calculated for the unknown gas.

b. If the flask was not dried before weighing, the final mass of flask would be higher, thus the mass of gas would be higher as well. This would give a greater molar mass value than actual value.

c. If the stopper was put on the flask right before weighing, most of vapor has been lost to the atmosphere. Thus, mass of vapor would be lower and thus, the molar mass of gas would be found lower than the actual value.

d. If all the liquid has vaporized, no condensation has occured, the mass of vapor would be higher. Thus molar mass calculated would turn out to be higher than actual value.

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