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Homework Help: Gas Pressure - Fluid Column

  1. Jul 4, 2017 #1
    1. The problem statement, all variables and given/known data
    I need to find the amount of H2 being lost to water by being stored underneath a column of water. About 16 mL of H2 is made and it displaces the column by 16 cm. However the total amount of water is 30 ml/30 cm. I'm not sure which relationship to use to find the pressure of the gas. I hypothesize that that pressure of the H2 is equal to the liquid pressure exerted by the water column on top of it.

    2. Relevant equations
    p = ghd

    C = P/k
    k= 7.8 x 10^-4 mol/L*atm

    3. The attempt at a solution

    p = ghd = (9.8m/s^2 x 0.30 m x 1000 kg/m^3)
    = 2940 Pa
    = 0.0290 atm

    C = P/k = (0.0290 atm)/(7.8 x 10^-4 mol/l*atm) = 2.263 x 10^-5 mol/L

    2.263 x 10^-5 mol/L x 0.016 L = 3.62 x 10^-7 moles H2 lost
  2. jcsd
  3. Jul 5, 2017 #2


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    Staff: Mentor

    What do you mean by "lost"?

    In typical situation "stored under water" doesn't mean there is "water on top of the gas". It rather means water seals the gas so that it can't escape nor mix with the air.

    Is really the 30 mL of water kept as a perfect column with an exact height of 30 cm?

    What about the ambient (atmospheric pressure)?

    Impossible to comment without knowing answers to these questions (most of them are related to your experimental setup).
  4. Jul 5, 2017 #3
    What is preventing the H2 from rising in the column?

    If you are using Henry's law, you need to use the absolute pressure, not just the hydrostatic part of the pressure. Is there a lid on the water column, or can the H2 diffuse to the top of the water and escape into the atmosphere?
  5. Jul 5, 2017 #4
    Hey there,

    I assumed by "lost" they meant how much gas dissolved in the water. But yes, water seals the gas. The gas also displaces the water by 16 ml as it is generated via electrolysis of water.

    The water appears to remain at 30 ml (which when measured with a ruler corresponds to 30 cm on the vessel I am required to use).

    The atmospheric pressure is 1 atm.
  6. Jul 5, 2017 #5

    H2 does rise in the column. If we continue generating H2, it escapes as bubbles. However, we stop hydrolysis when the gas starts to bubble. It also raises the height of the water by 16 cm (corresponding to 16 mL H2 generated).

    Yes, I am attempting to use Henry's law. There is no lid on the column. Would absolute pressure be hydrostatic + atm pressure?
  7. Jul 5, 2017 #6


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    Then I would simply assume the atmospheric pressure of 1 atm.
  8. Jul 5, 2017 #7
    Can you tell me why? Would it hurt to add the fluid pressure too?
  9. Jul 5, 2017 #8


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    Staff: Mentor

    Without knowing the exact setup it is hard to tell what is the additional pressure. A lot depends on the geometry.

    Besides, 30 cm means at most 3% error.
  10. Jul 5, 2017 #9
    Also, if there's not a lid on top, all the gas will eventually diffuse through the water and be released into the air.
  11. Jul 5, 2017 #10
    There is a drawing of the setup at the link. I don't have access to the equipment right now. The H2 gets up used up quickly to run a motor. Not much time for the gas to escape.
  12. Jul 5, 2017 #11
    I added a picture below. Thank you.
  13. Jul 5, 2017 #12
    I added a picture below. Thanks for your help!
  14. Jul 5, 2017 #13
    I don't see any picture.
  15. Jul 10, 2017 #14


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    Science Advisor
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    2017 Award

    He uploaded to an image site and provided a link in post #10. https://ibb.co/jSf6Ka
  16. Jul 10, 2017 #15
    Sorry. The figure just doesn't make sense to me.
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