Gen Chem- Ksp HELP!! please!

[orthdent786]

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This is a problem from the Kaplan Book...

Question- The solubility of Fe(OH)3 in an aqueous solution was determined to be 4.5 X 10^-10 mol/L. What is the vlaue of the Ksp for Fe(OH)3?

SOLUTION-- Fe(OH)3 (s) ---> Fe^3+ (aq) + 3OH- (aq)

So they made a balanced equation I understand that, but why did they put a +3 on the Fe?

So 1 mol of Fe(OH)3 dissociates--> 1 mole of Fe^3+ and 3 mol of OH- are produced. Ksp= (Fe^3+) (OH-)^3 then...

they write--- (OH-)= 3(Fe^3+) so where (what step)/ why does the coefficient get put infront of the Fe now?

If anyone can help it will be greatly appreciated!!! thanks in advance =)

 

[fancymylotus]

This is a problem from the Kaplan Book...

Question- The solubility of Fe(OH)3 in an aqueous solution was determined to be 4.5 X 10^-10 mol/L. What is the vlaue of the Ksp for Fe(OH)3?

SOLUTION-- Fe(OH)3 (s) ---> Fe^3+ (aq) + 3OH- (aq)

So they made a balanced equation I understand that, but why did they put a +3 on the Fe?

So 1 mol of Fe(OH)3 dissociates--> 1 mole of Fe^3+ and 3 mol of OH- are produced. Ksp= (Fe^3+) (OH-)^3 then...

they write--- (OH-)= 3(Fe^3+) so where (what step)/ why does the coefficient get put infront of the Fe now?

If anyone can help it will be greatly appreciated!!! thanks in advance =)

In order for the charges to balance, since each hydroxide has a charge of -1, the iron has to be in the +3
Also, Fe(OH)3 means 3OH's for every one Fe. Does the Ksp end up equaling 27X^4?

 

[orthdent786]

yes it does, ok so i understand that part but i still dont get how they just put the 3 in front of Fe^3+ in (OH-)=3(FE^3+)??

 

[Jejton]

What they are saying is that the concentration of OH- is 3 times that of Fe3+.

 

[Jejton]

Let concentration of Fe = x, then the concentration of OH = 3x.

 

[orthdent786]

thanks so much guys!! makes things alot clearer =)