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2515 Molar Mass of a Liquid

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    One of the most important applications of the Ideal Gas Law is the experimental determination of the molar masses (or "molecular weights") of gases. In fact, the method used in this experiment, the Dumas method, was the first widely used method for determining molar masses and was extremely important in the development of chemical theory.

    The density of a gas is given by the ideal gas equation as


    where M is the molar mass of the gas. Rearranging this equation to obtain molar mass gives


    Thus, the molar mass of a gas can be determined by measuring the temperature, pressure, mass, and volume of a substance in its gaseous phase.

    The ‘unknown’ liquids used in this experiment have boiling points less than 100°C. The liquid sample will be placed in a flask and then vaporized by heating the flask in a water bath. We will assume that the temperature of the vapor produced is the same as the temperature of the warm water bath that surrounds the flask (thermal equilibrium). An excess initial quantity of liquid sample is used so that its vapor will be more than enough to fill the flask. As a result, the vaporizing sample forces the air out of the flask as the vapor itself fills the flask. Any excess vapor also leaves the flask. Thus, after a few minutes, the flask contains only the vapor. The pressure of the vapor in the flask must equal the pressure of the atmosphere at the time (obtained from the barometer reading in the lab), since the flask is not closed off against atmospheric air. In addition, the volume of the vapor equals the volume of the flask. The flask is then removed from the warm water bath and the sample is allowed to cool and condense back into the liquid phase. The mass of the vapor is measured by weighing the condensate remaining in the flask.


    !!Wear your safety goggles!!

    Use tongs or mitts to handle hot flasks and beakers.

    Dispose of organic liquids in organic waste

    Most of the liquids that are used in this experiment will be evaporated into the atmosphere. Any excess organic liquids need to be disposed of in the organic waste container.

    3.0 CHEMICALS AND SolutionS



    Approximate amount


    "Unknown" liquid sample


    7 mL


    Dithizone dye crystal


    1 small piece






    250-mL Florence flask

    To hold liquid sample during heating


    Aluminum foil

    To cover the Florence flask


    Copper wire

    To secure the aluminum foil cover on the flask


    Ring stand with iron ring


    Crucible tong

    To hold and transport heated glassware


    Wire gauze

    To support beaker on an iron ring


    Bunsen burner

    To heat substances


    Utility clamps (2)

    To secure flask while being heated; to secure thermometer



    To measure temperature of water


    Rubber stopper with hole

    To support thermometer during use


    500-mL Graduated cylinder

    To measure total volume of the 250-mL Florence flask



    Liquid sample in this experiment may be any of the following: acetone, isopropyl alcohol, ethyl acetate, or cyclopentane. Additional liquids may also be added at the request of the instructor.

    1. Obtain a liquid sample from your instructor. Your instructor may choose to issue the liquid as an unknown sample. Record the name of the liquid sample in your data recording sheet, or the identification code of the unknown.
    1. Obtain a clean, dry 250-mL Florence flask, put one small crystal of dithizone dye and loosely cover the flask with a square of aluminum foil. Secure the aluminum foil by tying a copper wire. Weigh the flask with the cover and copper wire on it. In your data recording sheet, record mass to at least the nearest 1 milligram (0.001 grams).
    1. Carefully remove the copper wire and cover from the flask and add a small sample of liquid unknown - about 3 mL - to the flask. Swirl to dissolve the dithizone dye in the unknown liquid. Replace the cover and crimp it tightly around the neck of the flask. Secure the foil with the copper wire. Avoid leaving any loose foil or plastic wrap hanging down that might collect moisture from the water bath during the experiment.
    1. Use a pin to make a tiny pinhole in the center of the foil cover. This is done to allow the air and excess vapor to leave the flask, maintaining a pressure

    equal to atmospheric pressure.

    1. Prepare a water bath by filling a 1-L beaker with about 600 mL of tap water. Add a few boiling chips. Place it on a wire gauze on an iron ring attached to a ring stand. Adjust the height of the ring to make sure that the hottest part of the flame of the Bunsen burner touches the bottom of the beaker. The beaker can be secured with an oversize ring to prevent it from falling over. (If the beaker of water is too far away from the flame, it will take a very long time for the water to heat up!)
    1. Clamp the flask at its very top and suspend it in the water bath on the ring stand. Without allowing any water to get into the flask, push the flask as deep into the bath as possible, since the entire vaporized sample should be surrounded by water.
    1. Begin heating the bath with Bunsen burner. Mount the thermometer in the bath so that it is as close to the unknown as possible. Leave the thermometer in the bath throughout the warming time.
    1. Occasionally stir the water in the water bath with a stirring rod. Monitor the temperature of the water bath with a thermometer. Maintain the temperature of the water at around 95oC, or just below boiling throughout this experiment. Add additional water as needed to maintain the level of water.

    Alternatively, a different setup may be used with hot plates instead of Bunsen burners. Check with your instructor as to the specific procedure to use.

    1. Continue heating and watch the green color of the dithizone disappear – this indicates that the liquid sample has completely vaporized. If the water starts to boil while the liquid is evaporating, record this observation in your data recording sheet.

    It is very important that you keep the sample submerged during the vaporization process - do not take the flask out and put it back in the bath before the vaporization is complete, or it may jeopardize the accuracy of your results.

    1. Keep the flask in the boiling water for three minutes after the last of the unknown liquid vaporizes to ensure that the vapor is in complete thermal equilibrium with the boiling water in the bath. Record the temperature of the boiling water indicated on your thermometer. Turn off the Bunsen burner.
    1. Carefully remove the flask from the water bath by raising the clamp on the ring stand. The clamp may be hot; use thermal gloves or paper towels to protect from heat, if necessary. Let the flask hang on the ring stand until it has cooled to room temperature. During the wait, read the barometric pressure in the lab and record it.
    1. When the flask has cooled, dry the outside of the flask thoroughly using paper towels. Make sure there are no water droplets on the outside of the aluminum foil cover. Weigh the flask (with the condensed unknown liquid inside) with the aluminum foil cover and copper wire still in place. Record this mass in your data recording sheet.
    1. For a second trial, add about 2 - 3 mL of the same unknown to the flask

    Replace the cover (make sure it is the same cover!), and repeat steps 5-12. Use the same water bath as before - since the water is already warm, it will take less time to heat up.

    1. You have now recorded temperature, pressure, and weight of the sample for two separate trials. Volume must be measured next. This is NOT the same as the "250 mL" marked on the flask! To determine the volume, remove the cover and dump out the sample into the organic waste. Then, rinse out the flask and fill it to the brim with tap water. Pour this water into a 500-mL graduated cylinder and measure the volume to the nearest milliliter. You need to determine the volume of the expanded vapor in the flask, not just recording the volume marking on the flask.


    Last Name

    First Name


    Partner Name(s)


    6.1 Data

    Identification Code of Unknown

    Mass of the dry flask, dithizone crystal, foil, and copper wire


    Atmospheric Pressure (from barometer)

    Volume of flask (determined from your measured volume of water used to fill flask)



    Trial 1

    Trial 2

    Temperature of water bath


    Mass of flask, dithizone, foil, copper wire, and condensate


    Mass of condensate


    6.2 Observations

    1. Calculate a separate value of molar mass for each trial. Calculate the average value and the percent difference between the two values. If you trust one result more than the other, state which one and explain your reasoning.

    1. Obtain the identity of the unknown from your instructor. How does this compare with your experimental data? Calculate the percent error.


    1. Knowing the exact volume of the unknown liquid sample initially placed in the flask is not important. Why?

    1. Why do you have to immerse the flask as deep into the water bath as possible?

    1. When doing a second trial, why should you not dump out the condensate remaining in the flask before adding more sample?

    1. There are some experimental errors that could lead to a higher or lower calculated molar mass compared to the true value. In each case below, determine whether the experimental value would be higher or lower than the true value of the molar mass of the compound analyzed. Justify each of your answers with supporting rationale or equations.
    1. Air was not completely swept out of the flask by the unknown liquid after it had completely vaporized, so that the gas in the flask was a mixture of air and unknown.
    1. The volume of the flask at 100°C was greater than that at room temperature because of expansion of the glass. (The expansion of the glass is 1.4 ×10−5 deg–1. That is, the volume of the flask increases by 1.4 × 10−3 percent per degree rise in temperature.)

    1. The vapor was not an ideal gas. (Hint: Remember that the temperature of the vapor was not far above the boiling point of the unknown liquid. This fact should tell you the direction of the deviation from ideality.)

    1. Although the unknown liquid completely vaporized, the gas in the flask never reached the temperature of the boiling water.

    2515 Molar Mass of a Liquid is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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