Unscored C/P question 23

[deleted647690]

When I answered this question, I didn't really think through it. I just thought of PV = nRT, higher volume = lower pressure. I'm a little confused about the concept behind this. Is it asking what the pressure was before the liquid moved up the tube? Since it says that the pressure on the inside builds as the liquid rises until the inside pressure = outside pressure.

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[aldol16]

When you have a buret like this, you're basically measuring air pressure versus pressure inside the buret, in the trapped space. So when you evacuate the headspace in the buret, you remove all the gas pressure inside the buret. So the atmospheric pressure pushing on the surface of the fluid inside the beaker will cause the level of the water inside the buret to rise (since it's rising against a vacuum). When you produce H2 gas, the gas goes into the headspace and now you have that gas pressing down on the fluid inside the buret. If the fluid inside the buret is still higher than the fluid in the beaker, this means that the atmospheric pressure on the surface of the fluid inside the beaker is still winning out and is pushing the fluid up the buret.

 

[deleted647690]

When you have a buret like this, you're basically measuring air pressure versus pressure inside the buret, in the trapped space. So when you evacuate the headspace in the buret, you remove all the gas pressure inside the buret. So the atmospheric pressure pushing on the surface of the fluid inside the beaker will cause the level of the water inside the buret to rise (since it's rising against a vacuum). When you produce H2 gas, the gas goes into the headspace and now you have that gas pressing down on the fluid inside the buret. If the fluid inside the buret is still higher than the fluid in the beaker, this means that the atmospheric pressure on the surface of the fluid inside the beaker is still winning out and is pushing the fluid up the buret.

Thanks! So this is similar to how a thermometer works then, I guess I would find more information on this in the fluids chapter of my TBR books

 

[aldol16]

Thanks! So this is similar to how a thermometer works then, I guess I would find more information on this in the fluids chapter of my TBR books

It's not how a thermometer works - a thermometer works by the expansion of the liquid inside the thermometer. That liquid doesn't ever actually contact the outside world - it only exchanges energy with it.

It is, however, how a Torricellian barometer works:

 

[deleted647690]

It's not how a thermometer works - a thermometer works by the expansion of the liquid inside the thermometer. That liquid doesn't ever actually contact the outside world - it only exchanges energy with it.

It is, however, how a Torricellian barometer works:

Oh that was dumb of me haha

 

[rainbooww]

When I did the sample test the 2nd time, I actually got it wrong.
I was confused by the confusing description from the last paragraph, extensive words were used to describe how the water was drawn up to 50ml, and then the Q saying now the level inside the buret is "only" 27cm higher than the beaker level. This seemed to me the level has decreased overall inside the buret (due to production of H2), that must mean the pressure has increased.
I am so frustrated over this poorly worded question...

 

[BerkReviewTeach]

Unfortunately, poorly worded questions are not unusual in the science world. There are times you have to envision what they are describing rather than read about it, which is true here.