When species combine to produce a coordination complex, the equilibrium constant

Question

When species combine to produce a coordination complex, the equilibrium constant for the reaction is called is the formation constant, K_f. For example, the ion, Fe^2+, can combine with the cyanide ion, CN^-, to form the complex [Fe(CN)_6]^4- according to the equation Fe^2+(aq) + 6CN^-(aq) [Fe(CN)_4]^4- (aq) where K_1 = 4.21 Times 10^15. The average human body contains 5.50 L of blood with a Fe^2_ concentration of 3.10 Times 10^-15 M, if a person ingests 9.00 mL of 17.0 mM NaCN, what percentage of iron[III] in the blood would be sequestered by the cyanide ion?

Explanation / Answer

Fe^2+ + 2CN^- = Fe(CN)2
moles Fe^2 in blood = 5.50 L x 3.10x10^-5 mol/L = 1.71x10^-4 moles = 0.171 mmoles Fe^2+ present
moles CN^- ingested = 0.009 L x 17 mmol/L =0.153 mmoles
moles Fe^2+ sequestered by 0.153 moles CN^- = 1/2 x 0.153 = 0.0765 moles (see mole ratio of 2:1 in eq.)
% Fe^2+ sequestered = (0.0765moles/0.171 moles) x 100 = 44.7%

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