GChem problem in Kaplan BB, page 280...

[Cofo]

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Okay, I must be bad at math or something.

If you will, please help me with the Example question in Kaplan BB 2009-2010 edition....page 280.

Why does it say
Ksp = [Fe3+][OH-]^3
[OH-] = 3[Fe3+]

I thought for every 1 mole of Fe3+ produced, 3 mole of OH- is produced???

 

[loveoforganic]

Yep, so you have 3 times as much OH- as you have Fe3+. You're just confusing yourself.

 

[Baylor2011]

This confused me as well. Since the base concentration is cubed...for every one OH ion, you have 3 Fe ions

 

[Cofo]

Yep, so you have 3 times as much OH- as you have Fe3+. You're just confusing yourself.

Yes, I realize that, but then it says

Ksp = [Fe3+] (3[Fe3+])^3 = 27[Fe3+]^4

It's like the book is saying that they are multiplying the concentration of Fe3+ by 3 ....even though there is supposed to be 3 times as much OH-.
Why are they multiplying Fe3+ by 3?

 

[Cofo]

This confused me as well. Since the base concentration is cubed...for every one OH ion, you have 3 Fe ions

Thanks, this makes more sense now...I think.😕

So since I have [OH-]^3 ....do I have to try and get rid of the cubed sign, by taking the cubed root of both -OH and Fe3+ ???
And then I'll end up with [OH-] = [Fe3+]^3 ...which is really the same as 3[Fe3+] ???
ehhhhhhhhh............

 

[UCB05]

Yes, I realize that, but then it says

Ksp = [Fe3+] (3[Fe3+])^3 = 27[Fe3+]^4

It's like the book is saying that they are multiplying the concentration of Fe3+ by 3 ....even though there is supposed to be 3 times as much OH-.
Why are they multiplying Fe3+ by 3?

Like loveoforganic said, you're confusing yourself

y=3x means that for every x you have 3 times as much y. [OH-] = 3[Fe3+] means that for every Fe3+ ion you have 3 times as much OH- ion.

You don't have to get rid of the cubed sign yet.
Since [OH-] = 3[Fe3+], that is subbed into the equation, thus
Ksp = [Fe3+][OH-]^3
Ksp = [Fe3+](3[Fe3+])^3
Ksp = [Fe3+](27)[Fe3+]^3
Ksp = 27[Fe3+]^4