To verify whether a solutions contains iron(III) cations, thiocyanate (SCN^-) is

Question

To verify whether a solutions contains iron(III) cations, thiocyanate (SCN^-) is often used as test indicator. If SCN^- is added to a sample containing Fe^3+(aq), a blood-red solution is formed due to the appearance of the complex [Fe(NCS)(H_2O)_5]^2+ or {Fe(SCN)}^2+. The complex formation however does not proceed to completion, but the resulting compound establishes an equilibrium with its reactants Fe^3+(aq) and SCN^-(aq) (eq. 1): Fe^3+(aq) + SCN^-(aq) doubleheadarrow {Fe(SCN)}^2+(aq) The corresponding equilibrium constant K_c is also referred to as complex formation constant K_f of iron thiocyanate. 1) During the course of establishing complex formation constants for various iron complexes, 3.20g of iron (III) perchlorate hexahydrate, Fe(CIO_4)_3 middot 6 H_2O (MW: 462.3), are dissolved in 100 mL distilled water, producing a slightly yellow solution. 50 mL of a 0.125 M solution of potassium thiocyanate KSCN are added to the Fe^3+(aq) solution. The color change from light-yellow to deep-red indicates the formation of the iron (III) thiocyanate complex {Fe(SCN)}^2+(aq). The solution is allowed to equilibrate for several hours. Then, a 10 mL aliquot of the complex solution is diluted by adding 90 mL of distilled water in preparation for an absorbance measurement. The recorded absorbance allows one to determine the {Fe(SCN)}^2+concentration of the diluted solution; the complex concentration is [{Fe(SCN)}^2+] = 2.20 mM. Use this information and calculate a value of the complex formation constant K_f. Show your work for full credit. 2) Upon closer inspection of the reaction mixture, it is noted that a precipitate has formed. The solid is separated by filtration, and its weight is determined as 0.247g. Judged by its appearance, the solid is identified as yellow iron oxide or iron (III) oxide-hydroxide monohydrate FeO(OH) middot H_2O, which might also be described as solid iron (III) hydroxide Fe(OH)_3(s) (MW: 109.9). Use this information, and recalculate the value of the complex formation constant K_f. Show you work for full credit. 3) Suggest a modified procedure that prevents the formation of iron oxide or iron hydroxide compounds.

Explanation / Answer

Fe^3+ + SCN- <==> Fe(SCN)2 +

moles of Fe^3+ added = 3.2gm/462.3g/mol = 6.9*10^-3 moles

Molarity of Fe^3+ in the total volume = 6.9*10^-3 moles/0.15L = 0.046 M

moles of SCN- added = 0.125 M * 0.05 L = 6.25*10^-3 moles

Molarity of SCN in the mixture = 6.25*10^-3 moles/0.15L = 0.042 M

As amount of SCN is less, it is the limiting reagent and it will be consumed completely.

Given that [Fe(SCN)2]^2+= 2.2 mM = 0.0022 M in 100 mL diluted soluiton.

[Fe(SCN)2^+] in the concentrated solution =0.022M

So, x = 0.022 M

[Fe^3+]eqm = 0.046-0.022 = 0.024 M

[SCN-] = 0.042- 0.022 = 0.020 M

Kf = [Fe(SCN)2+]/[Fe^3+][SCN^-]

    = 0.022/(0.024)(0.02) = 45.83

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moles of iron oxide =0.247g/109.9g/mol = 2.25*10^-3 moles

molarity in 150mL solution = 0.015 M

Mass of Fe that has reacted to form Fe(SCN)^2+ = 6.9*10^-3 moles - 2.25*10^-3 moles = 4.65*10^-3

Molarity of Fe that has reacted to form Fe(SCN)^2+ = 4.65*10^-3/0.15 = 0.031 M

Kf = 0.022 /(0.031)*(0.02) =35.48

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To prevent this use a buffer to adjust the pH of the solution in a acidic rahe where Fe^3+ will not precipitate as Iron hydroxide

Fe^3+ SCN- Fe(SCN)2+ initial 0.046 0.042 0 change -x -x x equilibrium 0.046-x 0.042-x x

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