Thyroxine \(\left(\mathrm{C}_{15} \mathrm{H}_{11} \mathrm{I}_{4}
\mathrm{NO}_{4}\right)\) is a hormone synthesized by the thyroid gland and used
to control many metabolic functions in the body. A physiologist determines the
mass percent of thyroxine in a thyroid extract by igniting \(0.4332
\mathrm{~g}\) of extract with sodium carbonate, which converts the iodine to
iodide. The iodide is dissolved in water, and bromine and hydrochloric acid
are added, which convert the iodide to iodate.
(a) How many moles of iodate form per mole of thyroxine?
(b) Excess bromine is boiled off and more iodide is added, which reacts as
shown in the following equation:
$$
\mathrm{IO}_{3}^{-}(a q)+6 \mathrm{H}^{+}(a q)+5 \mathrm{I}^{-}(a q)
\longrightarrow 3 \mathrm{I}_{2}(a q)+3 \mathrm{H}_{2} \mathrm{O}(l)
$$
How many moles of iodine are produced per mole of thyroxine? (Hint: Be sure to
balance the charges as well as the atoms.) What are the oxidizing and reducing
agents in the reaction?
(c) The iodine reacts completely with \(17.23 \mathrm{~mL}\) of \(0.1000
\mathrm{M}\) thiosulfate as shown in the following unbalanced equation:
$$
\mathrm{I}_{2}(a q)+\mathrm{S}_{2} \mathrm{O}_{3}^{2-}(a q) \longrightarrow
\mathrm{I}^{-}(a q)+\mathrm{S}_{4} \mathrm{O}_{6}^{2-}(a q)
$$
What is the mass percent of thyroxine in the thyroid extract?