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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: VexedPig on May 02, 2019, 01:31:59 AM

Title: Is this question Possible????
Post by: VexedPig on May 02, 2019, 01:31:59 AM
Question 9: Answer BOTH parts (a) and (b).
The Kc at a particular temperature for the following reaction is 1.6x10-6 M.
2NOCl(g)      ⇌         2NO(g) + Cl2(g)
Calculate the concentrations of all the species at equilibrium for the following original mixtures:

a)   3.0 mol pure NOCl in a 3.0 Litre container
b)   2.5 mol NO and 1mol Cl2 in a 1Litre container


Title: Re: Is this question Possible????
Post by: sjb on May 02, 2019, 02:16:45 AM
Any thoughts of your own on how to start?
Title: Re: Is this question Possible????
Post by: VexedPig on May 02, 2019, 06:09:24 AM
Quote from: sjb on May 02, 2019, 02:16:45 AM
Any thoughts of your own on how to start?
Tried doing ICE method a hundred times and get some weird cubic equation at the end of it. I just want someone to acknowledge I'm not going completely insane.
Title: Re: Is this question Possible????
Post by: mjc123 on May 02, 2019, 08:06:37 AM
Why don't you show your working so we can see whether you're insane or not?
Yes, you do get a cubic equation. But in this case you can use the fact that Kc is very small to predict that the equilibrium is well over to the left. For part a, using the ICE table, I get
Kc = 4x3/(1-2x)2, where x is the equilibrium concentration of Cl2. Is this what you get?
But since K is so small, we can try making the simplification that x << 1, so (1-2x) ≈ 1. Then it's easy to solve, and you can (should) then check whether the assumption x << 1 is true.