TBR Gen Chem Equilibrium

[leathersofa]

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I have provided the question and Ksp values for reference.

71. The maximum [Ca2+] in a pH = 14 solution would be which of the following values?
A. 4.3 x 10^8 M Ca2+
B. 4.3 x 10^-2 M
C. 4.3 * 10^ -6 M
D. 4.3 x 10^-10 M Ca2+

KSP VALUES
CaF2: 4.8 * 10^-11
CaS04: 6.1 * 10^-6
CaC03: 9.1 * 10^-9
Ca(OH)2: 4.3 * 10^-6
Ca3(PO4)2: 1.3 * 10^-32

Answer: C

The answer explanation says how the "maximum calcium ion concentration at pH = 14 can be determined using the solubility product of calcium hydroxide(Ksp=[Ca2+][OH-]2)."

Why does TBR immediately assume that the product must be Ca[OH]2 ?? Couldn't the OH- react with the other given calcium compounds to form another calcium compound? Why is the product assumed to be Ca[OH]2?
Thank you.

 

[Romz]

I have provided the question and Ksp values for reference.

71. The maximum [Ca2+] in a pH = 14 solution would be which of the following values?
A. 4.3 x 10^8 M Ca2+
B. 4.3 x 10^-2 M
C. 4.3 * 10^ -6 M
D. 4.3 x 10^-10 M Ca2+

KSP VALUES
CaF2: 4.8 * 10^-11
CaS04: 6.1 * 10^-6
CaC03: 9.1 * 10^-9
Ca(OH)2: 4.3 * 10^-6
Ca3(PO4)2: 1.3 * 10^-32

Answer: C

The answer explanation says how the "maximum calcium ion concentration at pH = 14 can be determined using the solubility product of calcium hydroxide(Ksp=[Ca2+][OH-]2)."

Why does TBR immediately assume that the product must be Ca[OH]2 ?? Couldn't the OH- react with the other given calcium compounds to form another calcium compound? Why is the product assumed to be Ca[OH]2?
Thank you.

No...?

We are given a pH. Automatically we should be thinking OH/H concentrations. We know Ca2+ does not participate in increasing the pH, so we look for a compound which has Ca bound to a OH group. We see Calcium Hydroxide and know that out solution must have been a dissociation of Calcium Hydroxide. We solve for [OH] and find [Ca]. No need to assume OH reacts with other compounds or what not. The way I saw the question, TBR was testing to see if you understood how to tackle the problem. I don't believe all those compounds where [resent in solution.

You had a bit of flawed logic there. EVEN IF they where and we assume OH WOULD react with another compound, look at what it has to react with. OH being an anion would react with a cation. The only cation present is Ca2+. So if OH were to react, it would for Ca(OH)2 and just dissociate again regardless.

Hope that helps.

 

[leathersofa]

Thank you so much, your explanation really helped. I guess I needed a more detailed explanation, which you gave! so thank you very much!!

 

[Romz]

Haha no problem, good luck