A diatomic ideal gas with Cp = (7/2)R and Cv = (5/2)R is heated at constant pressure. Find the ratio dU: dQ: dW.

5: 7: 3
5: 7: 2
3: 7: 2
3: 5: 2

Correct answer: 5: 7: 2

Solution

For constant pressure heating: dQ = n*Cp*dT = (7/2)nRdT; dU = n*Cv*dT = (5/2)nRdT; dW = P*dV = nRdT. So dU:dQ:dW = (5/2)nRdT: (7/2)nRdT: nRdT = 5:7:2.

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