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Gases and Manometer

  1. Dec 2, 2005 #1
    1) In an open end manometer, one end of a U-tube filled with mercury is attached to a gas-filled container and the other end is open to the atmosphere. If the gas pressure in the container is less than atmospheric pressure


    a.Hg will be forced out of the open end of the U-tube.

    b.the Hg level will be higher in the arm connected to the container.

    c.the Hg level will be higher in the arm open to the atmosphere.

    d.the difference between the Hg levels in the two arms will be greater than 76 cm.

    2) The apparatus is called a closed-end manometer because the arm not connected to the gas sample is closed to the atmosphere and is under vacuum. What is the pressure (in mm Hg) of the gas inside this apparatus, where the arm open to the atmosphere has a higher mercury level than the arm connected to the gas container, if the outside pressure, Patm, is 745 mm Hg and the difference in mercury levels, delta h, is 25 mm Hg?

    a.25 mm Hg

    b.720 mm Hg

    c.745 mm Hg

    d.770 mm Hg

    Since the mercury in the open arm is higher, the pressure for the bulb/gas container is 745 + 25 mm Hg = 770mm Hg, or greater than the open arm's pressure?

    Thanks.
     
  2. jcsd
  3. Dec 3, 2005 #2

    Andrew Mason

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    1. is correct. I am not sure I understand the configuration of the manometer in 2. but your answer is correct. The gas pressure is balanced against atmospheric pressure plus 25mm of Hg.

    AM
     
  4. Dec 3, 2005 #3
    The apparatus is called a closed-end manometer because the arm not connected to the gas sample is closed to the atmosphere and is under vacuum. What is the pressure (in mm Hg) of the gas inside this apparatus, where the arm closed to the atmosphere has a higher mercury level than the arm connected to the gas container, if the outside pressure, Patm, is 745 mm Hg and the difference in mercury levels, delta h, is 25 mm Hg?

    Since the mercury in the closed arm with the vacuum is higher, the pressure for the bulb/gas container is 745 + 25 mm Hg = 770mm Hg, or greater than the open arm's pressure?

    Sorry, I just wanted to make some changes, but the answer is still correct, right?

    Could someone explain to me what is the vacuum's purpose in the manometer?
     
    Last edited: Dec 3, 2005
  5. Dec 3, 2005 #4

    lightgrav

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    Unless the gas container is balanced to atmospheric Pressure (by a piston, say), the outside P_atm has nothing to do with the Pressure in the closed system.

    The pressure in the vacuum-side arm is zero - that is the purpose of evacuating one side - so the difference of Hg column heights is a direct measurement of ABSOLUTE Pressure in the container.
     
  6. Dec 4, 2005 #5

    Andrew Mason

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    As has been pointed out, the purpose of the closed arm is to eliminate atmospheric pressure so that the mercury column measures just gas pressure rather than gas pressure plus atmospheric. With the change in the question, the answer is a) 25 mm.

    AM
     
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