Ksp Solubility question from KBB

[mashinka]

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Yo, so those of you out there with the 2008 edition its on pg 920 of the review book.

Question: The solubility of Fe(OH)3 in aq. solution is 4.5x10^-10 ml/L. What is the value of Ksp for Fe(OH)3?

Solution: assume equil. formula is Fe(OH)3 --> Fe3+ + 3OH-

Ksp = [Fe3+][OH-]^3

[OH-]= 3[Fe3+] 😕 where the hell does that come from?

[Fe3+] = 4.5x10^-10 ml/L *also, wtf? I thought that was the solubility for Fe(OH)3

Ksp = [Fe3+](3[Fe3+])^3 = 27[Fe3+]^4 --then just plug and chug

Thanks!!!!!

 

[Troyvdg]

Yo, so those of you out there with the 2008 edition its on pg 920 of the review book.

Question: The solubility of Fe(OH)3 in aq. solution is 4.5x10^-10 ml/L. What is the value of Ksp for Fe(OH)3?

Solution: assume equil. formula is Fe(OH)3 --> Fe3+ + 3OH-

Ksp = [Fe3+][OH-]^3

[OH-]= 3[Fe3+] 😕 where the hell does that come from?

[Fe3+] = 4.5x10^-10 ml/L *also, wtf? I thought that was the solubility for Fe(OH)3

Ksp = [Fe3+](3[Fe3+])^3 = 27[Fe3+]^4 --then just plug and chug

Thanks!!!!!

lol I remember coming across this and saying WTF .
If you have chads videos, definitely watch them for Keq Ksp Ka and Kb.


but, I think your confusing solubility and Ksp.
I like to think of solubility as the amount dissolved in a volume, while ksp is a constant, for the max compound that can be dissolved.

so they give you the solubility of Fe(OH)3 which is the amount of that compound they dissolved.

So since we are looking for the ksp, we can set up our Ksp expression as:

Ksp = [Fe3+][OH-]^3 --> we are looking for the concentration of Fe3+ and OH- ions.

They give you the amount of Fe(OH)3 dissolved. And, for every Fe(OH)3 dissolved, you'll have 1 Fe3+ ion and 3 OH- ions that form.

Since, only 1 Fe3+ forms we know that, The [Fe3+] formed is equal to the amount of Fe(OH)3 dissolved.

so we can say [Fe3+] = 4.5 x 10^-10

and for every Fe3+, there are 3 OH- formed. So, [OH-] = 3(4.5 x 10^-10)

therefore, we can set up the equation:

Ksp=[4.5 x 10^-10][3 x (4.5 x 10^-10)]^3

plug and chug and it'll get you 1.1 x 10^-36

hope you could understand all that 👍

 

[mashinka]

totally helpful!
Thanks sir