1mole of an ideal gas A(Cv,m=3R) and 2mole of an ideal gas B are (CV,m=32R) taken in a container and expanded reversible and adiabatically from 1litreto4litre starting from initial temperature of 320K. ΔE or ΔU for the process is:
A
−240R
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B
240R
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C
480R
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D
−960R
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Solution
The correct option is D−960R AverageCv,m=n1Cv,m1+n2Cv,m2n1+n2=2R
For adiabatic process dU=dW dTT=−RCv,m(dVV)n1CV,m1dT+n2Cv,m2dT=−(n1RT+n2RT)×dVVInT2T1=−12In(V2V1)⇒T2=320×(14)12=160KΔU=(n1CV,m1+n2CV,m2)ΔT=−960R