KSP question

[deleted74029]

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If the dissociation reaction is :
Fe(OH)3----><-----Fe³+ + 3OH-

Ksp = [Fe³+][OH-]³

I don't understand how [OH-] = 3[Fe³+]

I would think it would be 3[OH] = [Fe³+]

My book says its the former.

 

[dwc929]

If the dissociation reaction is :
Fe(OH)3----><-----Fe³+ + 3OH-

Ksp = [Fe³+][OH-]³

I don't understand how [OH-] = 3[Fe³+]

I would think it would be 3[OH] = [Fe³+]

My book says its the former.

For every mole of Fe3+, you make 3 moles of OH

so it's actually 3[Fe3+] = [OH]...seems kinda weird at first but try it w/ #s

i.e. if for every 1 apple, you have 3 oranges...your # of oranges will be 3*(apples)

 

[Adnama]

i.e. if for every 1 apple, you have 3 oranges...your # of oranges will be 3*(apples)[/quote]


I am so confused.

Can you please explain this further? I am sitting here with 1 apple and 3 oranges- why don't I have 3 apples?

 

[sleepy425]

ok, so let's go with the apples and oranges example. for every apple, i have three oranges.

so say I have 100 apples. how many oranges do I have? 300, right? how did you get that? well, you multiplied the number of apples you had by 3. so that means 3*apples=oranges.

basically, the thing that's tripping you up is the fact that you're thinking that you have more of the one that you multiply by three. however, look at this:

say I have NaCl, which would be 1:1 dissociation:

so you would have [Na+]=[Cl-]
this one's obvious

now for BaCl2, which would be twice as many chlorides as bariums:

so you would have 2[Ba++]=[Cl-]
what does this mean physically? well, if I have 10 barium ions in a given volume, then plugging 10 into the equation means I have 20 chloride ions. that makes sense since one BaCl2 will dissociate into one barium and two chloride ions.

so for Fe(OH)3, which would be three times as many hydroxides as irons:

so you would have 3[Fe3+]=[OH-].
what does this mean physically? well, if I have 10 ferric ions in a given volume, then plugging 10 into the equation means I have 30 hydroxide ions. this makes sense since one Fe(OH)3 will dissociate into one ferric and three hydroxide ions.

 

[Adnama]

ok, so let's go with the apples and oranges example. for every apple, i have three oranges.

so say I have 100 apples. how many oranges do I have? 300, right? how did you get that? well, you multiplied the number of apples you had by 3. so that means 3*apples=oranges.

basically, the thing that's tripping you up is the fact that you're thinking that you have more of the one that you multiply by three. however, look at this:

say I have NaCl, which would be 1:1 dissociation:

so you would have [Na+]=[Cl-]
this one's obvious

now for BaCl2, which would be twice as many chlorides as bariums:

so you would have 2[Ba++]=[Cl-]
what does this mean physically? well, if I have 10 barium ions in a given volume, then plugging 10 into the equation means I have 20 chloride ions. that makes sense since one BaCl2 will dissociate into one barium and two chloride ions.

so for Fe(OH)3, which would be three times as many hydroxides as irons:

so you would have 3[Fe3+]=[OH-].
what does this mean physically? well, if I have 10 ferric ions in a given volume, then plugging 10 into the equation means I have 30 hydroxide ions. this makes sense since one Fe(OH)3 will dissociate into one ferric and three hydroxide ions.

That is a great explanation! ... thanks so much!! (guess I can put the fruit away)