# Ideal gas - monatomic or diatomic?

by shyguy79
Tags: diatomic, ideal, monatomic
 P: 102 1. The problem statement, all variables and given/known data 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 2. Relevant 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 3. The attempt at a solution No idea where to start... help please!
HW Helper
P: 6,189
 Quote by shyguy79 1. The problem statement, all variables and given/known data 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 2. Relevant 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 3. The attempt at a solution No idea where to start... help please!
Hi shyguy79!

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?
 P: 102 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?
 HW Helper P: 6,189 Ideal gas - monatomic or diatomic? Yep. That is correct. What would Cp/nR be for a monatomic gas? And for a diatomic gas?
 P: 102 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?
 HW Helper P: 6,189 Right. :)
 P: 102 Thank you so much! I've been stuck on this for ages!
 P: 102 Just one question though what equation would be referenced for dividing Cp by nR?
 HW Helper P: 6,189 Not sure what you mean...? You've used 3 of your relevant equations and solved for "f".
 P: 102 doh! Yeah, just noticed - thanks again!

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