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Ideal gas - monatomic or diatomic?

  • Thread starter shyguy79
  • Start date
  • #1
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Homework Statement



5 moles of an idea gas at 300K at a pressure of 1.00 x 10^5 Pa is heated to 500K at constant pressure. The amount of heat transferred is 29.1kJ.

Determine whether a gas is monatomic or diatomic through consideration of the values of the molar heat capacity at constant pressure C sub c,m and at C sub v,m

Homework Equations


PV = nRT
C = dQ/dT
Cp = Cv + nR
Cv = f/2 nR where f = 3 for monatomic gas and f = 5 for diatomic gas

The Attempt at a Solution


No idea where to start... help please!
 

Answers and Replies

  • #2
I like Serena
Homework Helper
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Homework Statement



5 moles of an idea gas at 300K at a pressure of 1.00 x 10^5 Pa is heated to 500K at constant pressure. The amount of heat transferred is 29.1kJ.

Determine whether a gas is monatomic or diatomic through consideration of the values of the molar heat capacity at constant pressure C sub c,m and at C sub v,m

Homework Equations


PV = nRT
C = dQ/dT
Cp = Cv + nR
Cv = f/2 nR where f = 3 for monatomic gas and f = 5 for diatomic gas

The Attempt at a Solution


No idea where to start... help please!
Hi shyguy79! :smile:

In a constant pressure process you have Cp=dQ/dT.
Can you calculate Cp from that?
What would you get if you divide Cp by nR?
 
  • #3
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Where dQ = 29.1x10^3 and dT = 200K then Cp = 145.5 J K^-1

Is nR = 5 x 8.314 = 41.57 ???? but why would you do this?

so Cp/nr = 3.5 ???? How does this relate?
 
  • #4
I like Serena
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Yep. That is correct.

What would Cp/nR be for a monatomic gas?
And for a diatomic gas?
 
  • #5
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Cp = (f/2 +1) nR so then Cp/nR = (f/2 +1)

so for a monatomic gas... 3/2 + 1 = 2.5
for a diatomic gas... 5/2 +1 = 3.5

so the gas is diatomic?
 
  • #6
I like Serena
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Right. :)
 
  • #7
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Thank you so much! I've been stuck on this for ages!
 
  • #8
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Just one question though what equation would be referenced for dividing Cp by nR?
 
  • #9
I like Serena
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Not sure what you mean...?

You've used 3 of your relevant equations and solved for "f".
 
  • #10
102
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doh! Yeah, just noticed - thanks again!
 

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