What Is the Volume of Dry Hydrogen Gas at STP?

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Discussion Overview

The discussion revolves around calculating the volume of dry hydrogen gas at standard temperature and pressure (STP) based on a given sample collected over water. Participants explore the implications of different definitions of STP and the calculations involved in determining the volume of the gas.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a calculation for the volume of dry hydrogen gas, using the total pressure and vapor pressure of water to find the dry pressure.
  • Another participant questions the definition of STP, suggesting that there is confusion regarding the temperature and pressure values used in the calculations.
  • Some participants clarify that STP is defined as 0°C and 1 atm (760 torr), while others mention that 25°C (298 K) is often used in practice, referring to it as standard ambient temperature and pressure (SATP).
  • A participant acknowledges a mistake in their calculations related to the definition of STP, indicating that this confusion is common among learners.

Areas of Agreement / Disagreement

Participants express disagreement regarding the definition of STP, with some asserting that it is 0°C while others reference 25°C as commonly used in calculations. The discussion remains unresolved regarding the correct application of these definitions in the context of the problem.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about temperature and pressure definitions, as well as the potential for confusion between STP and SATP. The calculations presented depend on these definitions, which are not universally agreed upon in the context of the problem.

gurerd
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Homework Statement


A 32.0 mL sample of hydrogen is collected over water at 20°C and 750.0 torr pressure. What is the volume of the dry gas at STP? (vapor pressure of water at 20°C = 17.5 torr)


Homework Equations



Ptot = Pdry + Pwet

P1V1/T1 = P2V2/T2

The Attempt at a Solution



Pdry = Ptot - Pwet = 750 - 17.5 = 732.5 torr

V1 = P2V2T1/P1T2 = (732.5 torr)(32 mL)(298K)/(760 torr)(293 K) = 31.3 mL

The answer in the answer key gives 28.7 mL
 
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What is STP?
 
STP is standard temperature and pressure -- which I used
T = 25.0 °C ( 298 Kelvin) and
P = 1 atm of pressure (760 torr)

Did I use these in the correct variables? Perhaps that is where I am confused?
 
gurerd said:
STP is standard temperature and pressure -- which I used
T = 25.0 °C ( 298 Kelvin) and
P = 1 atm of pressure (760 torr)
That's SATP. STP is T = 0°C.
 
Ah, what a silly mistake. It works out now!

Thanks so much DrClaude! I appreciate your time.
 
gurerd said:
Ah, what a silly mistake. It works out now!
You would be surprised how commen this mistake is. This is probably what SteamKing was alluding to. the main problem is that most thermodynamic is for 298 K, but STP was defined as 273 K.

gurerd said:
Thanks so much DrClaude! I appreciate your time.
You're welcome!
 

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