gchem problem

[joefosho315]

Hey guys, I've got a question:

I'm given that Keq 96.2 at 450C for P(Cl5) (aq) --> P(Cl3) (aq) + Cl2 (g) and that the equilibrium concentration of Cl2 = 3.50M and P(Cl5) = 3.50M. The question wants me to figure out what the equilibrium concentration of P(Cl3) is. It's actually a fairly simple problem to solve, but I'm having conceptual problems understanding the context of the problem. I don't understand how Cl2 can have a different concentration than PCl3, since these products both form at a 1:1 ratio. From what I understand, for every 1 mol of P(Cl5) that dissociates in the solution, you get 1 mol of P(Cl3) and 1 mol of Cl2. Therefore, in a conceptual sense, I thought that the concentration of Cl2 (3.50M) would have to equal the concentration of P(Cl3). But this is not the case, according to the solution that goes through the Keq = product/reactant calculation method. So can someone tell me where I'm going wrong in my thinking?

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[AaBbZz]

What was the books given solution? Maybe it has something to do with redox reaction i'm not sure.

 

[TimeforDAT]

when doing keq equations do you include gases in the equation too or is it just aqueous ?

 

[bryanttttttta]

I got this!

This is not a stoichiometry problem. This is a type of problem where they give you "initial concentrations" (like random #'s). After awhile, those concentrations would move to "equilibrium concentration" (Cl2 = 3.50M and P(Cl5) = 3.50M). Do you remember Q to determine which way the rxn will shift?

 

[infernobutterfl]

are you sure the keq is 96.2 and that the concentration given is at equi.? cuz if that is the case, you will be getting a ton of pcl3 (if my understanding of how to solve the problem is correct).

 

[joefosho315]

Yes, the Keq is at 96.2, I guess somewhat expected given the fact that the temperature is quite high at 450C. So yes, the amount of PCl3 at equilibrium is quite high as well. Bryant, you were saying that this problem gives me initial amounts of some of the products and reactants. This makes sense to me, as then in this case the amount of PCl3 could differ from Cl2. I guess I just assumed improperly from this question that in the beginning, PCl5 was simply placed into water. If the latter was the case, then surely the amount of Cl2 and PCl3 would be the same, correct?