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# A2 Equilibrium Question :s watch

1. Hey, I wonder if someone could help me with this question. I am struggling to grasp exactly what to do

PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)

at 250 oC, in a 2.0dm3 vessel, there is 0.15 mol of PCl3 and 0.090 mol of Cl2.
Kc = 0.19 mol dm-3 at 250 oC

Q) Calculate the amount of PCl5 at equilibrium, and hence the mass of PCl5 at equilibrium.

So far I've figured out that the concentration of PCl5, [PCl5], is 0.0178 mol dm-3. But what now? I'm so confused!

2. (Original post by gabby07)
Hey, I wonder if someone could help me with this question. I am struggling to grasp exactly what to do

PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)

at 250 oC, in a 2.0dm3 vessel, there is 0.15 mol of PCl3 and 0.090 mol of Cl2.
Kc = 0.19 mol dm-3 at 250 oC

Q) Calculate the amount of PCl5 at equilibrium, and hence the mass of PCl5 at equilibrium.

So far I've figured out that the concentration of PCl5, [PCl5], is 0.0178 mol dm-3. But what now? I'm so confused!

I'm a little puzzled by the information you've given, can you clarify: do you mean 0.15 mol of PCl3 and 0.090 mol of Cl2 were put into the reaction vessel or were they in those quantities at equilibrium?
I'm sure you need to know the quantity of at least one of the reactants used initially and one of the quantities at equilibrium, otherwise you can't work it out.
3. (Original post by pineneedles)
I'm a little puzzled by the information you've given, can you clarify: do you mean 0.15 mol of PCl3 and 0.090 mol of Cl2 were put into the reaction vessel or were they in those quantities at equilibrium?
I'm sure you need to know the quantity of at least one of the reactants used initially and one of the quantities at equilibrium, otherwise you can't work it out.
Sorry about that! It is supposed to be the equilibrium mixture.
4. (Original post by gabby07)
Sorry about that! It is supposed to be the equilibrium mixture.
That's fine, do you know how much of each were put in, initially?
5. (Original post by pineneedles)
That's fine, do you know how much of each were put in, initially?
To answer the other part of your question, I also thought that since the question asks for the amount present at equilibrium we don't need to worry about initial quantities? Not sure though. As I say, I can only manage to get as far as finding the concentration of PCl5. I am wondering if dividing this concentration by the volume of the vessel (2dm3) could give me the number of moles? Does that work?
6. That is all the information provided also.
7. I think you just need to rearrange to Kc expression to find the conc [PCl5], then work from there.
8. (Original post by gabby07)
To answer the other part of your question, I also thought that since the question asks for the amount present at equilibrium we don't need to worry about initial quantities? Not sure though. As I say, I can only manage to get as far as finding the concentration of PCl5. I am wondering if dividing this concentration by the volume of the vessel (2dm3) could give me the number of moles? Does that work?
Sorry, sorry, never mind that. In all of these types of questions I've done, it's been to calculate Kc rather than this. You need initial to calculate kc but not for this.

Your value for the concentration of PCl5 is fine, just multiply that by the volume it's in to get the moles.

N = CV !
9. (Original post by pineneedles)
Sorry, sorry, never mind that. In all of these types of questions I've done, it's been to calculate Kc rather than this. You need initial to calculate kc but not for this.

Your value for the concentration of PCl5 is fine, just multiply that by the volume it's in to get the moles.

N = CV !
Grrr can't believe I didn't think of that! Thanks so much

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