2. Three students, Mason, Amir, and Kamyar, measured an empty Erlenmeyer flask and a
   tight-fitting stopper and found the mass to be 62.371 g. The students then carefully
   measured out 4.4 mL of a volatile liquid, poured the liquid into the flask, and heated the
   flask containing the liquid in a 101.1°C boiling water bath. As soon as all the liquid had
   vaporized, the students covered the flask with the stopper and set it aside to cool. Then,
   after a few minutes, they very quickly removed and replaced the stopper and reweighed
   the flask. The mass of the flask, stopper, and condensed vapor was 63.088 g. At the
   conclusion of the experiment, the students rinsed out the flask, filled it with water to the
   top, replaced the tight-fitting stopper (being sure there were no air bubbles), and obtained
   a volume of 261.9 mL. The barometric air pressure that day was 733 mm Hg.

(a) Calculate the pressure of the vapor inside the flask (in atm) after the vapor had

cooled and the students opened the flask momentarily.

(b) What was the mass of the vapor in the flask?

(c) Calculate the mass of 1 mole of the vapor.

(d) Explain how each of the following errors in the laboratory procedure would affect

the calculation of the molecular mass.

(1) The volatile liquid contained nonvolatile impurities.

(2) The students removed the flask from the water bath before all of the liquid

had vaporized.

(3) There was a hole in the stopper.

(4) There were a few drops of water left on the flask from the water bath when

the final mass was taken.

Answer

Begin this problem by numbering the problem and listing the critical information, getting rid of
superfluous words, and clarifying the situation in your mind.

2. Given: 62.371 g = Erlenmeyer flask + stopper
   4.4 mL of volatile liquid
   101.1°C boiling water bath = 374.1 K
   63.088 g = flask, stopper, condensed vapor
   261.9 mL = volume of stoppered flask
   733 mm Hg = barometric pressure

Then, number each section; restate the question in as few words as possible; list the necessary
information; write down any generic formulas; substitute the data from the problem into the
generic formulas; do the math; and underline or box your answer.

(a) Restatement: Pressure inside of flask after vapor cooled and the flask was opened

momentarily.

1.

733

760

mm Hg (^10964)
mm Hg

atm = atm

The Gas Laws