Heat and Thermodynamics 4 Question 11

13. A gas mixture consists of 3 moles of oxygen and 5 moles of argon at temperature T. Considering only translational and rotational modes, the total internal energy of the system is

(2019 Main, 11 Jan I)

(a) 12RT

(b) 15RT

(c) 20RT

(d) 4RT

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Solution:

  1. Internal energy of a gas with f degree of freedom is

U=nfRT2, where n is the number of moles.

Internal energy due to O2 gas which is a diatomic gas is

U1=n1f1RT2=3×52RT

(n1=3 moles, degree of freedom for a diatomic gas f1=5 )

Internal energy due to Ar gas which is a monoatomic gas is

U2=n2f2RT2=5×32RT

(n2=5 moles, degree of freedom for a monoatomic gas f2=3 )

Total internal energy =U1+U2

U=15RT



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