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 iron(II) 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)_6)^4 (aq) where K_f = 4.21 times 10^45 This reaction is what makes cyanide so toxic to human beings and other animals The cyanide ion binds to the iron that red Mood cells use to carry oxygen around the body, thus interfering with the blood's ability to deliver oxygen to the tissues It -s this toxicity that has made the use of cyanide in gold mining controversial Most states now ban the use of cyanide in leaching gold out of low-grade ore. The average human body contains 5.50 L of blood with a Fe^2i concentration of 1.50 times10^-5 M If a person ingests 8.00 mL. of 10.0 mM NaCN, what percentage of iron(ll) in the blood would be sequestered by the cyanide ion?

Explanation / Answer

According to the given equation above, 1 mole of Fe2+ combines with 6 moles of CN- to form a complex

moles of Fe2+ = 1.5 x 10^-5 M x 5 L = 7.5 x 10^-5 mol

moles of NaCN = 0.01 M x 0.008 L = 8 x 10^-5 mol

[Fe(CN)6]4- formed = 8 x 10^-5/6 = 1.33 x 10^-5 mol

Fe2+ remaining = 6.17 x 10^-5 mol

% Fe2+ reacted with CN- = 1.33 x 10^-5 x 100/7.5 x 10^-5 = 17.73%

% Fe2+ remained free of CN- = 82.27%

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