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Entropy and the partition function for nitrogen

  • #1
89
1

Homework Statement



I'm attempting to calculate the translational entropy for N2 and I get a value of 207.8 J/Kmol. The tabulated value is given as 150.4 and I am stumped as to why the decrepancy.
T = 298.15 K and P = 0.99 atm and V = 24.8 L
R = 8.314 J/Kmol[/B]

Homework Equations


Strans = R ln[(2ΠmkT/h2)3/2*V*e5/2/Na][/B]

The Attempt at a Solution


I plugged in all the correct(?) values and cannot figure out what I'm doing wrong.
[/B]
 
Last edited:

Answers and Replies

  • #2
DrClaude
Mentor
7,153
3,288
What value did you use for the mass?
 
  • #3
89
1
28 amu =
4.65E-26 kg
 
  • #4
89
1
My calculated value is off by 30% which is huge. I rechecked the constants I used and the units :
h = 6.63E-34 m2kg/s
k
B = 1.38E-23 JK-1
NA
= 6.02E+23
T
= 298.15 K
V
= 24.79 L
I really am stumped. I'm using Excel to do the calculation and I've checked it several times and
ln[(2ΠmkT/h2)3/2*Ve5/2/Na] calculates to 25 when it "should" be about 18 and change.
I don't know where to go with this. All the other entropy contributions to the entropy from the total partition function were right on the mark except this one. HELP!
 
  • #5
89
1
My calculated value is off by 30% which is huge. I rechecked the constants I used and the units :
h = 6.63E-34 m2kg/s
k
B = 1.38E-23 JK-1
NA
= 6.02E+23
T
= 298.15 K
V
= 24.79 L
I really am stumped. I'm using Excel to do the calculation and I've checked it several times and
ln[(2ΠmkT/h2)3/2*Ve5/2/Na] calculates to 25 when it "should" be about 18 and change.
I don't know where to go with this. All the other entropy contributions to the entropy from the total partition function were right on the mark except this one. HELP!
I've also found, thanks to Excel, that the descrepancy involes either a factor of .01 inside the power expression
(2ΠmkT/h2)3/2 or .001 times the expression [(2ΠmkT/h2)3/2*Ve5/2/Na] but I still don't know what it could be.
 
  • #6
TSny
Homework Helper
Gold Member
12,403
2,838
Be sure to convert liters to SI units.
 
  • #7
89
1
Wow. Thanks a pile dude(?). What an embarassing oversite on my part! (Homer Simpson Duh!) All this time, I'm in my 50's, and I'm thinking liter is, or absentmindedly thinking it is, the SI unit for volume. It's all about the units, man.
 

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