Gases Homework Help - Calculating Moles & Explaining Water Vapour Pressure

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Discussion Overview

The discussion revolves around a homework problem involving the calculation of moles of hydrogen gas produced in a reaction between magnesium and hydrochloric acid, as well as explanations regarding water vapor pressure and the adjustment of water levels in a gas collection tube. The scope includes theoretical and conceptual aspects of gas behavior and pressure in a laboratory setting.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant presents a calculation for the number of moles of hydrogen gas produced, stating their answer as 0.00127 moles.
  • Another participant explains that the vapor pressure of water is relevant because water vapor will be present in the gas collected, especially since the gas is bubbled through water.
  • A question is raised about the implications of adjusting the water levels in the collection tube, particularly regarding the relationship between the pressure inside the tube and the barometric pressure outside.
  • It is noted that when the water levels are equal, it indicates that the pressures inside and outside the collection tube are equal, allowing for further calculations regarding the partial pressures of gases present.

Areas of Agreement / Disagreement

Participants generally agree on the relevance of vapor pressure and the need for equal water levels to assess pressure relationships, but the specific calculations and interpretations of pressure dynamics remain open to discussion.

Contextual Notes

There are unresolved aspects regarding the calculations of moles and the exact contributions of different gases to the total pressure in the system, as well as assumptions about the negligible presence of HCl vapor.

vijay123
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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.

regards
vijay
 
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2)explain why the vapour pressure of water was given.

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.

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.
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)?
 
so are you syaing that the pressure inside and outside the collecting tube is 750mmhg?
 
vijay123 said:
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.
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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