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Topic: Solubility constant  (Read 4644 times)

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Offline Firehchicken

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Solubility constant
« on: October 23, 2017, 11:17:19 AM »
Hi, the answer to the question below is C. I do not understand why; isn't the concentration equal to the square root of the KsP???



Offline sjb

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Re: Solubility constant
« Reply #1 on: October 23, 2017, 11:22:23 AM »
What is the expression you have developed for Ksp?

Offline Firehchicken

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Re: Solubility constant
« Reply #2 on: October 23, 2017, 12:08:36 PM »
What is the expression you have developed for Ksp?

Ksp = [Zn2+][CN-]

is it not? Since it dissociated in a one to one ratio can we assume that their concentrations are equal.

Offline mjc123

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Re: Solubility constant
« Reply #3 on: October 23, 2017, 12:12:28 PM »
Is it dissociated in a 1:1 ratio?

Offline Firehchicken

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Re: Solubility constant
« Reply #4 on: October 23, 2017, 10:41:59 PM »
Is it dissociated in a 1:1 ratio?

Right, I understand it is dissociated in a 1:2 Zn:CN ratio, but i still can't figure out the answer.

By this logic it must follow that (assuming the concentrations of Zn and Cn are x) : x^3 = Ksp = 2.56 × 10^-13.

Taking the cube root does not give me an answer of 8.0 × 10^-5.

Back-tracking, cubing 8.0 × 10^-5 gives me a value of 5.12×10^-13, which is twice the Ksp value. There's something not quite right here ??? ??? ???

Offline Flatbutterfly

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Re: Solubility constant
« Reply #5 on: October 24, 2017, 12:41:56 AM »
Zn(CN)2(s) ⇋ Zn^2+ + 2CN^-
Ksp = [Zn^2+][CN^-]^2 = 2.56×10^-13
Let x mol of Zn(CN)2 dissolve then [Zn^2+] = x;  [CN^-] = 2x
Ksp = (x)(2x)^2 = 2.56×10^-13
4x^3 = 2.56×10^-13
x = 4×10^-5 M
The answer key is incorrect

Offline Borek

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Re: Solubility constant
« Reply #6 on: October 24, 2017, 02:54:33 AM »
x = 4×10^-5 M
The answer key is incorrect

What about

Quote
[CN^-] = 2x
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Flatbutterfly

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Re: Solubility constant
« Reply #7 on: October 24, 2017, 12:12:34 PM »
Yes - it occurred to me this morning that [CN^-] = 2x = 8x10^-5M but I could only see half the question posted.

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