Q130CP - Chemistry: Molecular Nature Of Matter And Change

Question

Thyroxine (C₁₅H₁₁I₄NO₄) 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 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 unbalanced equation: 

${\mathbf{IO}}_{\mathbf{3}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{H}}^{\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{I}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{\to }{\mathbf{I}}_{\mathbf{2}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\left(I\right)$ 

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 mL of 0.1000 M thiosulfate as shown in the following unbalanced equation:

${\mathbf{I}}_{\mathbf{2}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{S}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{3}}^{\mathbf{2}\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{\to }{\mathbf{I}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{S}}_{\mathbf{4}}{\mathbf{O}}_{\mathbf{6}}^{\mathbf{2}\mathbf{-}}\left(\mathrm{aq}\right)$ 

What is the mass percent of thyroxine in the thyroid extract?

Step-by-Step Solution

Answer

      a. The number of moles of iodate formed per mole of thyroxine are 4 mol.

      b.The moles of iodine produced are 12 mol.

      c.The mass percent of thyroxine is 12.86%.

Step 1: Determination of moles of iodate

The ignition of thyroxin with sodium carbonate, converts the iodine to iodide. The reaction for this process is as follows,

${\mathbf{C}}_{\mathbf{5}}{\mathbf{H}}_{\mathbf{11}}{\mathbf{I}}_{\mathbf{4}}\left(s\right)\mathbf{+}{\mathbf{Na}}_{\mathbf{2}}{\mathbf{CO}}_{\mathbf{3}}\left(s\right)\mathbf{\to }\mathbf{4}{\mathbf{I}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{other}\mathbf{ }\mathbf{products}$ 

 

The iodide is reacts with bromine and hydrochloric acid which converts iodide to iodate. The reaction for this process is,

${\mathbf{I}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{Br}}_{\mathbf{2}}\left(I\right)\mathbf{+}\mathbf{HCI}\left(\mathrm{aq}\right)\mathbf{\to }{\mathbf{IO}}_{\mathbf{3}}^{\mathbf{-}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}\mathbf{other}\mathbf{ }\mathbf{products}$ 

Thus, the number of moles of iodate formed per mole of thyroxine are 4 mol.

Step 2: Determination of moles of iodine

The given reaction is,

${\mathbf{IO}}_{\mathbf{3}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{H}}^{\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{I}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{\to }{\mathbf{I}}_{\mathbf{2}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\left(I\right)$ 

 

The balanced equation for the above reaction is,

$\mathbf{2}{\mathbf{IO}}_{\mathbf{3}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{12}{\mathbf{H}}^{\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{10}{\mathbf{I}}^{\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{\to }\mathbf{6}{\mathbf{I}}_{\mathbf{2}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{6}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\left(I\right)$

 

For the above reaction, the oxidising agent is IO₃^- and the reducing agent is I^-.

 

The number of moles of iodine produced from 1 mol of thyroxine are,

$\mathbf{1}{\mathbf{molC}}_{\mathbf{15}}{\mathbf{H}}_{\mathbf{11}}{\mathbf{I}}_{\mathbf{4}}{\mathbf{NO}}_{\mathbf{4}}\mathbf{\times }\frac{\mathbf{4}{\mathbf{moll}}^{\mathbf{-}}}{\mathbf{1}\mathbf{mol}\mathbf{ }{\mathbf{C}}_{\mathbf{15}}{\mathbf{H}}_{\mathbf{11}}{\mathbf{I}}_{\mathbf{4}}{\mathbf{NO}}_{\mathbf{4}}}\mathbf{\times }\frac{\mathbf{4}\mathbf{mol}\mathbf{ }{\mathbf{IO}}_{\mathbf{3}}^{\mathbf{-}}}{\mathbf{4}\mathbf{mol}\mathbf{ }{\mathbf{IO}}_{\mathbf{3}}^{\mathbf{-}}}\mathbf{\times }\frac{\mathbf{6}{\mathbf{molI}}_{\mathbf{2}}}{\mathbf{2}{\mathbf{molIO}}_{\mathbf{3}}^{\mathbf{-}}}\mathbf{=}\mathbf{12}{\mathbf{molI}}_{\mathbf{2}}$ 

 

Thus, the moles of iodine produced are 12 mol.

Step 3: Determination of mass percent of thyroxine

The given unbalanced reaction is,

${\mathbf{I}}_{\mathbf{2}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{S}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{3}}^{\mathbf{2}\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{\to }{\mathbf{I}}^{\mathbf{-}}\mathbf{ }\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{S}}_{\mathbf{4}}{\mathbf{O}}_{\mathbf{6}}^{\mathbf{2}\mathbf{-}}\left(\mathrm{aq}\right)$ 

 

The balanced equation for the above reaction is,

${\mathbf{I}}_{\mathbf{2}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}\mathbf{2}{\mathbf{S}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{3}}^{\mathbf{2}\mathbf{-}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{\to }\mathbf{2}{\mathbf{I}}^{\mathbf{-}}\mathbf{ }\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}{\mathbf{S}}_{\mathbf{4}}{\mathbf{O}}_{\mathbf{6}}^{\mathbf{2}\mathbf{-}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}$ 

 

 

Moles of thiosulfate are,

$$\begin{gathered} \mathbf{Moles}\mathbf{=}\mathbf{molarity}\mathbf{\times }\mathbf{volume} \\ \mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{1}\mathbf{mol}\mathbf{/}\mathbf{L}\mathbf{\times }\mathbf{17}\mathbf{.}\mathbf{23}\mathbf{mL}\mathbf{\times }\frac{\mathbf{1}\mathbf{L}}{\mathbf{1000}\mathbf{mL}} \\ \mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{001723}\mathbf{mol} \end{gathered}$$ 

 

 

Moles of thyroxine are,

$\mathbf{0}\mathbf{.}\mathbf{001723}\mathbf{mol}\mathbf{ }{\mathbf{S}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{3}}^{\mathbf{2}\mathbf{-}}\mathbf{\times }\frac{\mathbf{1}{\mathbf{moll}}_{\mathbf{2}}}{\mathbf{2}\mathbf{mol}\mathbf{ }{\mathbf{S}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{3}}^{\mathbf{2}\mathbf{-}}}\mathbf{\times }\frac{\mathbf{2}{\mathbf{mollO}}_{\mathbf{3}}^{\mathbf{-}}}{\mathbf{6}{\mathbf{moll}}_{\mathbf{2}}}\mathbf{\times }\frac{\mathbf{2}\mathbf{moll}}{\mathbf{2}{\mathbf{mollO}}_{\mathbf{3}}^{\mathbf{-}}}\mathbf{\times }\frac{\mathbf{2}\mathbf{moll}}{\mathbf{2}{\mathbf{moll}}^{\mathbf{-}}}\mathbf{\times }\frac{\mathbf{1}{\mathbf{molC}}_{\mathbf{15}}{\mathbf{H}}_{\mathbf{11}}{\mathbf{I}}_{\mathbf{4}}{\mathbf{NO}}_{\mathbf{4}}}{\mathbf{4}\mathbf{moll}}\mathbf{=}\mathbf{7}\mathbf{.}\mathbf{179}\mathbf{\times }{\mathbf{10}}^{\mathbf{-}\mathbf{6}}\mathbf{ }\mathbf{mol}$ 

The mass of thyroxine is,

$$\begin{gathered} \mathbf{Mass}\mathbf{=}\mathbf{moles}\mathbf{\times }\mathbf{molar}\mathbf{ }\mathbf{mass} \\ \mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{=}\left(7.179\times {10}^{-5} \mathrm{mol}\right)\mathbf{\times }\mathbf{776}\mathbf{.}\mathbf{8}\mathbf{g}\mathbf{/}\mathbf{mol} \\ \mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{0557}\mathbf{\times }\mathbf{g} \end{gathered}$$ 

 

Now, the mass percent of thyroxine is,

$$\begin{gathered} \mathbf{Mass}\mathbf{\%}\mathbf{=}\frac{\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{thyroxine}}{\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{thyroid}}\mathbf{\times }\mathbf{100} \\ \mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{=}\frac{\mathbf{0}\mathbf{.}\mathbf{0557}\mathbf{g}}{\mathbf{0}\mathbf{.}\mathbf{4332}\mathbf{g}}\mathbf{\times }\mathbf{100} \\ \mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{=}\mathbf{12}\mathbf{.}\mathbf{86}\mathbf{\%} \end{gathered}$$ 

 

Thus, the mass percent of thyroxine is 12.86%.