GATE 2021 BT – Question 34
Two reactors contain ideal gases: R1 has 5 L at 9 atm, R2 has 10 L at 6 atm. Opening their connecting valve at constant temperature gives equilibrium pressure _______ atm.
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Correct answer: 7
Explanation
At constant temperature the total number of moles is conserved when the valve is opened, and $PV=nRT$ gives $n\propto PV$.
**Total $PV$ before mixing:**
$$P_1V_1+P_2V_2=9\times5+6\times10=45+60=105\text{ atm L}.$$
**After mixing** the gas occupies $V_1+V_2=15$ L at the same temperature:
$$P_f\times15=105\;\Rightarrow\;P_f=\mathbf{7\ atm}.$$