An ideal gas in a thermally insulated vessel at internal pressure = P1, volume = V1 and absolute temperature = T1 expands irreversibly against zero external pressure, as shown in the diagram. The final internal pressure, volume and absolute temperature of the gas are P2 , V2 and T2 , respectively. For this expansion,

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Pext = 0
w = 0 thermally insulated q = 0
∴ ΔU = 0 , hence T2 = T1
P1V1 = P2V2