What is the Relationship Between Gas Pressure and Fluid Column Height?

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Discussion Overview

The discussion revolves around the relationship between gas pressure and fluid column height, specifically in the context of hydrogen gas (H2) being generated and stored under a column of water. Participants explore the implications of this setup on the pressure of the gas and the amount of gas that may dissolve in the water.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant hypothesizes that the pressure of the H2 is equal to the liquid pressure exerted by the water column above it.
  • Another participant questions the meaning of "lost," suggesting that it may refer to the amount of gas dissolved in the water rather than gas escaping into the atmosphere.
  • Concerns are raised about the experimental setup, including whether the water column is maintained perfectly and the role of ambient atmospheric pressure.
  • Some participants emphasize the need to consider absolute pressure when applying Henry's law, which includes both hydrostatic and atmospheric pressure.
  • It is noted that if there is no lid on the water column, the gas may eventually diffuse into the atmosphere.
  • Participants discuss the geometry of the setup and its potential impact on pressure measurements, with one noting that a height of 30 cm could introduce a small error margin.
  • A participant mentions that the hydrogen gas rises and escapes as bubbles during generation, but the process is stopped before significant gas escape occurs.

Areas of Agreement / Disagreement

There is no consensus on the exact relationship between gas pressure and fluid column height, as participants express differing views on the implications of the experimental setup and the definitions of terms used. Multiple competing views remain regarding the interpretation of "lost" gas and the necessity of considering absolute pressure.

Contextual Notes

Participants highlight the importance of understanding the experimental setup, including the presence or absence of a lid and the geometry of the water column, which may affect pressure calculations. There are also references to potential errors in measurements due to the height of the water column.

krbs
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Homework Statement


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.

Homework 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
 
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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).
 
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?
 
Borek said:
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).

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.
 
Chestermiller said:
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?

Hello!

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?
 
krbs said:
I assumed by "lost" they meant how much gas dissolved in the water.

Then I would simply assume the atmospheric pressure of 1 atm.
 
Borek said:
Then I would simply assume the atmospheric pressure of 1 atm.
Can you tell me why? Would it hurt to add the fluid pressure too?
 
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.
 
Also, if there's not a lid on top, all the gas will eventually diffuse through the water and be released into the air.
 
  • #10
https://ibb.co/jSf6Ka
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.
 
  • #11
Borek said:
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.
I added a picture below. Thank you.
 
  • #12
Chestermiller said:
Also, if there's not a lid on top, all the gas will eventually diffuse through the water and be released into the air.
I added a picture below. Thanks for your help!
 
  • #13
krbs said:
I added a picture below. Thanks for your help!
I don't see any picture.
 
  • #14
Chestermiller said:
I don't see any picture.
He uploaded to an image site and provided a link in post #10. https://ibb.co/jSf6Ka
 
  • #15
scottdave said:
He uploaded to an image site and provided a link in post #10. https://ibb.co/jSf6Ka
Sorry. The figure just doesn't make sense to me.
 

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