Ideal gas law problems: Pascals vs Atms?

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Discussion Overview

The discussion centers around the application of the ideal gas law, specifically the use of different pressure units (atmospheres vs. pascals) in calculations involving gas properties. Participants explore unit conversions and the implications of using various unit systems in solving problems related to the ideal gas law.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions how to determine when to use pressure in atmospheres versus pascals, suggesting that both approaches (using different unit systems) could yield correct results.
  • Another participant emphasizes the importance of using a consistent unit system and ensuring that constants are in the same units to maintain accuracy in calculations.
  • A different viewpoint advocates for using SI units (mKgs) as the most reliable method, despite the potential for larger numbers in calculations.
  • It is noted that when using SI units, pressure should be in pascals, volume in cubic meters, and the ideal gas constant R should be 8.314 J/K-mol, while in another system, pressure is in atmospheres, volume in liters, and R is 0.0821 L-atm/K-mol.
  • Participants agree that conversion from other units (like torr or psi) to either system is possible, but the correct value of R must be used according to the chosen unit system.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of unit consistency and the possibility of using different unit systems, but there is no consensus on which system is preferable or more reliable.

Contextual Notes

The discussion highlights the need for careful unit conversion and the selection of appropriate constants, but does not resolve the best practices for unit selection in all scenarios.

Who May Find This Useful

Individuals studying or working with the ideal gas law, particularly those interested in the implications of unit systems in scientific calculations.

ddoctor
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Hi group,

Simple arithmetic problems using PV = nRT or NkT. How do you know when to use pressure units in atmospheres or pascals? For example, in 1 particular problem they give you a pressure in units of Torr, volume in cubic cm and temp in Kelvin and ask for the number of molecules. Couldn't you convert Torr to atm, cm to L OR Torr to Pascals and cm to m. In the first case you get n number of moles and multiply by avagadro's number to get molecules. In the second case you use NkT to get the number of molecules directly, probably easier, but aren't both approaches correct?
Thanks
Dave
 
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As long as all of your units are in the same system and you use the appropriate constants (in the same units) you will be fine. Just keep track of your units to ensure that your answers make sense.
 
My advice is:use SI-mKgs.The most correct and reliable.Sometimes involves big numbers,but,hey,where would the fun be in such problems??

Daniel.
 
When using SI units : P is in Pa (or N/m^2), V is in m^3, T is in Kelvin and R = 8.314 J/K-mol

When using the other units : P is in atm, V is in Liters, T is in Kelvin and R = 0.0821 L-atm/K-mol

You can you any system you like. Remember to use the right value of R in that system.

If you are given values in units other than the two above (eg : torr, psi, ml, cc, centigrade, fahrenheit), you can simply convert to either one system of units.
 

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