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AP Chemistry

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The molar mass of a volatile liquid (one which vaporizes easily) can be determined by a the DUMAS METHOD.' In this process a small amount of the liquid (a few drops) is placed in a clean flask. The mouth of the flask is covered with a piece of foil that has a small hole (pin hole) in it. The flask is heated in a boiling water bath until the liquid vaporizes completely. The air in the flask (and the excess vapor) escapes through the pinhole leaving the flask filed with the vapor. When the flask cools the vapor condenses and can be massed along with the flask. In this way the volume of the vapor (volume of the flask),temperature of the vapor(temperature of the water bath) pressure of the vapor (atmospheric pressure) and mass of vapor (mass of flask and condensed vapor minus the mass of the empty flask) can be determined. The molar mass of the gas can then be determined using the formula from this lab. Suppose that the original few drops of liquid added to the flask contained a nonvolatile (will not vaporized) contaminate. WHAT effect (if any) would this have on the experimentally determined molar mass of the liquid?

  • AP Chemistry -

    Didn't I answer this for you yesterday.

  • AP Chemistry -

    You did not completely vaporize the liquid when you heated it. Is that incorrectly high, incorrectly low, or unaffected by the following procedural changes?

  • AP Chemistry -


  • AP Chemistry -

    Can someone answer this:

    1) too high, B) too low or C) unaffected. In each case, explain how this result occurs

    a) after removing the flask from the water bath, the experimenter cools the flask to room temperature but does not dry it. The calculated molar mass will be: A) B) C)

    b) the flask is removed form the water bath containing vapor only, the experimenter cools the flask to room temperature and some vapour condenses inside the flask. The calculated molar mass will be: A) B) C)

    c) the flask volume is not measured; instead the experimenter assumes the flask volume to be exactly 125.00mL. The calculated molar mass will be: A) B) C)

    d) From the time the mass of the unused flask assembly (flask, foil, and elastic band) is recorded the flask is handled several times with oily fingers. The calculated molar mass will be: A) B) C)

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