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Gases homework help

  1. Jul 4, 2006 #1
    heres a question-(tough one)..i jus wunna check my answer..

    a small sample of magnesium was placed into a reaction flask as shown. HCL acid was added and the hydrogen gas generated wus bubbled through water into a gas collection tube.when the reaction reached completion, the gas collection tube wus nearly full. then gas was allowed to sit for several minutes. the collection tube wus then adjusted so that the level of water in the tube was equal to the level of water outside the tube. the vol. of gas was determined to be 32ml and the temp. of the water was 22(degress)celsisus. the barometric pressure in the lab. was 750mmhg. the vapour pressure of water at 22(degress)celsisus is 19.4mmhg.
    1)calculate the no. of moles of hydrogen gas produced.(my ans. is 0.00127moles).
    2)explain why the vapour pressure of water was given.
    3)explain why it was necessary to adjeust the level of the tube so that water levels inside and outside of it were the same.

  2. jcsd
  3. Jul 4, 2006 #2


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    Well, at 22°C there will be water vapor (and maybe some HCl, but that may be negligible) in the gas collected in the collection tube, especially since one is bubbling through water.

    What would one have to do if the water level in the collection tube was not the same? When the water levels are the same, what does that indicate about the pressure inside the collection tube in relationship to the barometric pressure in the lab (750mmHg)?
  4. Jul 4, 2006 #3
    so are you syaing that the pressure inside and outside the collecting tube is 750mmhg?
  5. Jul 10, 2006 #4


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    I don't have my calculator so I can't do a number crunch to check that

    You are given the partial pressure of water because the pressure in a system is a sum of everything in the system.
    HCl pressure + water vapour pressure = system pressure

    When the water levels match, you can say the pressures match. From there you can say the pressure inside your system is 750mm. Subtract your water pressure and you'll know your HCl gas pressure.
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