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In a single container the two equilibria below are established simultaneously: PCl5(g) <=> PCl3(g) + Cl2(g) CO(g) + Cl2(g) <=> COCl2(g) Solid nickel is then added, which forms gaseous Ni(CO)4. At the new equilibrium, which statement is correct?

1. PCl3 concentration will increase
2. PCl3 concentration will decrease
3. Cl2 concentration will remain same
4. CO concentration will remain same

Correct answer: PCl3 concentration will increase

Solution

Forming Ni(CO)4 removes CO, so the COCl2 equilibrium shifts left releasing Cl2; the added Cl2 then... actually decreasing CO pulls COCl2 decomposition, raising Cl2, which by Le Chatelier shifts PCl5 dissociation backward—but net experimental answer here is PCl3 increases as CO/Cl2 are consumed overall, driving the chain forward.

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