A student weighs an empty flask and stopper and finds the mass to be 53.256 g. S

Question

A student weighs an empty flask 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 degree C. After all the liquid is vaporized, she removes the flask from the bath, stoppers it, and lets it cool. After it is cool, she momentarily removes the stopper, then replaces it and weighs the flask and condensed vapor, obtaining a mass of 53.780 g. 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. What was the pressure of the vapor in the flask in atm? What was the temperature of the vapor in K? the volume of the flask in liters? What was the mass of condensed vapor that was present in the flask? How many moles of condensed vapor were present? What is the mass of one mole of vapor (Eq. 2)?

Explanation / Answer

a) since 760 mmHg = 1 atm

so 728 mmHg = 0.95789 atm

b) The temperature of vapor = 98.8 +273 = 371.8 K

    the volume of the flask = 231.1mL = 0.2311 L

c) m = 53.780 g - 53.256g = 0.524 g

e) To find out the molar mass we use the equation PV = (m/M) RT

hence M = m R T / P V

              = 0.524 g x 0.082057 L-atm/K mol x 371.8K / 0.95789atm x 0.2311L

              = 72 g/mol

d) now calculate no. of moles = 0.524g / 72.217 g/mol

                                           = 7.26 x 10-3 moles

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