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An engine undergoes an adiabatic expansion stroke. Immediately after combustion, pressure is 20 atm and volume is 25 cm³. After adiabatic expansion, the final volume is 200 cm³. Given: n = 5 mol, Cp/Cv = gamma = 4/3, R = 25/3 J/(mol*K). Which of the following is correct?

1. Pressure at the end of expansion stroke is 1.25 atm.
2. Change in temperature in expansion stroke is 0.6 K.
3. Work done on gas is 75 J during expansion stroke
4. All of the above

Correct answer: Pressure at the end of expansion stroke is 1.25 atm.

Solution

P2 = P1*(V1/V2)^(4/3) = 20*(25/200)^(4/3) = 20*(1/8)^(4/3) = 20/(8^(4/3)) = 20/16 = 1.25 atm. For temperature and work: use nCv*delta_T = -W, T1 = P1V1/(nR). T1 = (20*101325*25*10⁻⁶)/(5*25/3) = 50663/(41.67) = 1215.9 K approx... but with R=25/3 and using atm*cm³ consistently, delta_T = (P1V1 - P2V2)/((gamma-1)*nR). Only option (a) is definitively correct.

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