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For PCl5 <-> PCl3 + Cl2, the equilibrium mixture at a total pressure of 10 atm contains 2, 6 and 4 moles of PCl5, PCl3 and Cl2 respectively. If equilibrium is re-established at a total pressure of 50 atm (same temperature), what is the new number of moles of PCl5 at equilibrium?

1. about 4.73 mol
2. about 2.00 mol
3. about 6.00 mol
4. about 1.30 mol

Correct answer: about 4.73 mol

Solution

Kp = 10 atm from the first state; treating the system as 8 mol initial PCl5 dissociating by x at 50 atm gives 50*x²/((8-x)(8+x)) = 10, so x ~ 3.27 and PCl5 ~ 4.73 mol (more than before, as raising pressure suppresses dissociation).

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