Analía Bellizzi – Chemistry Classes

Ronald Reagan High School

AS- Finding the molar mass of a volatile liquid


We will heat a volatile liquid to evaporate it so that its vapor just fills a flask whose volume we can measure; we will also know the temperature and pressure of the vapor, and the mass of the vapor. With those data and the pressure measured using a barometer, we can calculate the molar mass of the liquid.


List the materials you used.



  1. Determine the mass of flask and foil cap and rubber band (This is done before adding the liquid so we can weigh the amount of vapor by the difference in mass from a later measurement)
  2. Make a small pin-hole in the foil cap.
  3. Add the volatile liquid. (About 5 mL is sufficient. It is not crucial to get exactly 5 mL)
  4. cover with the foil using the rubber band to hold it in place.  
  5. Prepare a water bath using a bigger beaker where the flask fits.
  6. Once the liquid disappears (all liquid vaporized already), measure water bath temperature before taking it out from the water bath.
  7. Place the flask inside the water bath. This will allow the liquid to vaporize.
  8. Heat up until the liquid disappears.
    • Keep as much of the flask under water as possible.
    • Heat the water bath gently. The assumption that the water bath and the vaporized liquid have the same temperature is only reasonable if the water is being heated gently (with the temperature not rising rapidly). Don’t let the water boil.
  9. Measure the atmospheric pressure  in the barometer on Mr. Torres’s Class or look up for the information online during the lab (tell your teacher you will be using your phone)
  10. Let the vapor condense in flask, for this, run cold water over flask.
  11. Determine mass of flask, foil cap, rubber band and condensed vapor
    • There will be less volatile liquid left than there was initially; that is what we expect. You put in more than enough liquid so its vapor would fill the flask and the excess left through the pinhole. The condensed vapor (i.e., the remaining liquid) has the same mass as the vapor which exactly filled the flask.
    • Be sure the foil cap is not wet from boiling water or condensed water vapor. Dry the neck of the flask, if necessary, keeping the foil cover on.
  12. Determine volume of flask The flasks are nominally 250 mL, but 250 mL is not an exact measurement; furthermore, it refers to the volume of liquid it holds in normal use, so it excludes the volume of the flask’s neck.
      • After you are finished with all determinations, rinse the flask with water
      • Now fill your flask (to overflowing) with tap water, and measure with a 500-mL graduated cylinder the volume it contains.

Data Tables

In the lab, we determined the volume of the gas by measuring the volume of water contained in the flask. we can also calculate it by measuring the mass of water contained in the full flask and then using the density at room temperature, calculate the volume of that liquid.

Lab Setup

Mr volatile liquid setup


Show all calculations and formulas you used during this lab in this section


Analysis Questions:

  1. Enumerate at least three possible error sources and how to avoid them in further labs.
  2. The compound we used was propanone. Write the Lewis structure of Propanone.
  3. Predict the hybridization of the three carbons in acetone.
  4. Give an estimated value for the angles in each bond for the three carbons
  5. Propanone is non polar. Based in the molecular structure (angles and hybridizations) explain why propanone is almost non polar.
  6. Which intermolecular forces are present in propanone?
  7. Why does propanone has a lower boiling point than water?
  8. Why do we use a water bath in this experiment?

Your lab report should have the following parts:

  • Title
  • purpose
  • materials
  • procedures
  • data tables
  • calculations
  • analysis questions