Question

In: Chemistry

Given the following information about Fe in aqueous solutions, what will be the ratio of (Fe(OH)2+)...

Given the following information about Fe in aqueous solutions, what will be the ratio of (Fe(OH)2+) to (Fe(OH)30) in solution at pH 6.0, 6.5, 7.0, 7.5, 8.0, 8.5?

Fe3+ + 2H2O = Fe(OH)2+ +2H+ logK = -5.7

Fe3+ + 3H2O = Fe(OH)30 + 3H+ logK = -13.1

Solutions

Expert Solution

Fe3+ + 2H2O = Fe(OH)2+ +2H+ logK = -5.7

K = [Fe(OH)2+ ][H+ ]^2/[Fe3+ ]            k = 10^(-5.7)

[Fe(OH)2+ ] = K[Fe3+ ] / [H+ ]^2

Fe3+ + 3H2O = Fe(OH)30 + 3H+ logK1 = -13.1

K1 = [ Fe(OH)30 ][H+ ]^3/[Fe3+ ]       k1 = 10^(-13.1)

[ Fe(OH)30 ] = K1[Fe3+ ]/[H+ ]^3

therefore

[Fe(OH)2+ ]/[ Fe(OH)30 ] =       K [H+ ]/ K1

at pH = 6 [H+] = 10^(-6)

[Fe(OH)2+ ]/[ Fe(OH)30 ] = (10^(-5.7)*10^(-6))/ (10^(-13.1))

[Fe(OH)2+ ]/[ Fe(OH)30 ]    = 25.12

at pH = 6.5

[Fe(OH)2+ ]/[ Fe(OH)30 ] = (10^(-5.7)*10^(-6.5))/ (10^(-13.1))

[Fe(OH)2+ ]/[ Fe(OH)30 ]    = 7.94

At pH = 7.0

[Fe(OH)2+ ]/[ Fe(OH)30 ] = (10^(-5.7)*10^(-7))/ (10^(-13.1))

[Fe(OH)2+ ]/[ Fe(OH)30 ]    = 2.511

at pH= 7.5

[Fe(OH)2+ ]/[ Fe(OH)30 ] = (10^(-5.7)*10^(-7.5))/ (10^(-13.1))

[Fe(OH)2+ ]/[ Fe(OH)30 ]    = 0.794

at pH = 8

[Fe(OH)2+ ]/[ Fe(OH)30 ] = (10^(-5.7)*10^(-8))/ (10^(-13.1))

[Fe(OH)2+ ]/[ Fe(OH)30 ]    = 0.2511

at pH = 8.5

[Fe(OH)2+ ]/[ Fe(OH)30 ] = (10^(-5.7)*10^(-8.5))/ (10^(-13.1))

[Fe(OH)2+ ]/[ Fe(OH)30 ]    = 0.0794


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