uJJiglment, yBu submit answers by question parts. The number of submissions rema

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uJJiglment, yBu submit answers by question parts. The number of submissions remaining for each question part only changes if you submit or c Assignment Scoring Your best submission for each question part is used for your score. 13. -14 points What pH is required to start precipitation of Fe(OH)3 from a 0.320 M solution of FeCh? Ksp of Fe(OH)3 = 4.0 x 10- 29 Submit Answer Save Progress Home My Assignments Extension Requesf WebAssign 4.00 1997-2017 Advanced Instructional Systems, Inc. All nghts reserved F6 FS F2 esc

Explanation / Answer

Expression for Fe(OH)3 dissociation is given by

Fe3+(aq) + 3 OH-(aq) -------> Fe(OH)3(s)

Solubility product, Ksp = [Fe3+][OH-]3

FeCl3 undergoes complete dissociation in solution, [Fe3+] = 0.320M
Precipitation occurs for [OH-]3 = Ksp/ [Fe3+]  
= 4.0 X 10-29/0.32  
= 1.25 X 10-28 M

Therefore, [OH-] = 1.07 X 10-10 M
We know, [OH-][H+] = Kw = 10-14
pH= -log( [H+] )
= - log(Kw/[OH-] )
= 14 + log([OH-] )
= 14 + log[1.07 X 10-10]
= 4.02

So the pH at which Fe(OH)3 starts to precipitates is 4.02

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