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### Question 11.13

Posted: Sun Jul 23, 2017 5:54 pm
"Write the reaction quotient Q for:
(a) 2 BCl3(g) + 2 Hg (l) --> B2Cl4 (s) + Hg2Cl2 (s)
(b) P4S10 (s) + 16 H2O (l) --> 4 H3PO4 (aq) + 10 H2S (aq)
(c) Br2 (g) + 3 F2 (g) --> 2 BrF3 (g)"

As I understand, Q is equal to [products]/[reactants]. However, the answer for (a) is 1/PBCl3^2, and I am not sure how to get that answer. I feel like I'm overthinking this one, but if anyone could help me out, that would be great. Thanks!

### Re: Question 11.13

Posted: Sun Jul 23, 2017 6:17 pm
The reason why the answer is 1/PBCl3^2 is because as stated in page 133 in the course reader, molar concentration of a pure substance (solid or liquid) does not change in a reaction
This means that solid + liquid are not included in K expression

As you can see in problem A, 2 BCl3(g) + 2 Hg (l) --> B2Cl4 (s) + Hg2Cl2 (s)
2 BCl3 is gas
2 Hg is liquid
B2Cl4 is solid
And Hg2Cl2 is solid

We know that Q = [products] / [reactants]
Therefore, the only thing left in the [product] is 1 and [reactant] is PBCl3^2