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Topic: Equilibrium concentration of $$Zn^{2+}$$ in $$[ZnCN_4^{2-}]$$  (Read 222 times)

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Win,odd Dhamnekar

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Equilibrium concentration of $$Zn^{2+}$$ in $$[ZnCN_4^{2-}]$$
« on: January 11, 2021, 09:21:11 AM »
Calculate the equilibrium concentration of Zn2+  in a Zn(CN)42-

Zn2+(aq)+ 4CN (aq) ZnCN42-(aq) ;  Kf =1×1018

Let x be the change in concentration as Zn2+ dissociates. Because the initial Zn2+ concentration is 0, the concentration at any times is x:

$$1.0\times10^{18}=\frac{[Zn(CN)_4]^{2-}}{[Zn^{2+}][CN^-]^4}=\frac{0.30-x}{x(4x)^4}$$

$$1\times 10^{18}*(256x^5)=0.30- x$$

Since x is very small in comparison to 0.30 M, drop x:
Solving this equation, we get x=6.51 × 10-5 M

« Last Edit: January 11, 2021, 11:40:49 AM by Win,odd Dhamnekar »
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mjc123

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Re: Equilibrium concentration of $$Zn^{2+}$$ in $$ZnCN_4^{2-}$$
« Reply #1 on: January 11, 2021, 11:02:36 AM »
Yes, but take care with some matters of presentation. Write the concentration of the complex as [Zn(CN)42-]. (I know we sometimes use square brackets in writing complexes, but this is a fairly simple one, and it's important to distinguish charges from exponents.) Also, there is no such species as "4CN". You should write [CN-] (don't forget the minus charge). [CN-] = 4x. We have discussed this before. Do you understand what the numbers mean in an equation?

Win,odd Dhamnekar

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Re: Equilibrium concentration of $$Zn^{2+}$$ in $$[ZnCN_4^{2-}]$$
« Reply #2 on: January 11, 2021, 11:49:42 AM »
As per your reply, i made the necessary changes in my answer. I think it now comply with the rules for the presentation in chemistry.
Any science consists of the following process.
1) See 2) Hear 3) Smell if needed 4) Taste if needed
5) Think 6) Understand 7) Inference take decision [Believe or disbelieve, useful or useless, healthy or unhealthy, cause or effect, favorable or unfavorable, practical or theoretical, practically possible or practically impossible, true or false or  any other required criteria]

mjc123

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Re: Equilibrium concentration of $$Zn^{2+}$$ in $$[ZnCN_4^{2-}]$$
« Reply #3 on: January 12, 2021, 05:03:57 AM »
There's another point that occurs to me. You say "Since x is very small in comparison to 0.30, drop x". It's a good idea to check such an assumption at the end of your calculation. You could simply add a line:
"6.51 x 105 << 0.30, therefore the assumption was valid."
It does no harm to write this explicitly in your answer; even if you don't, it's good to get into the habit of making that check in your own mind whenever you use such an assumption.