Problems Solve Each, Show your work for full credit.
20. (8 pts) An iodometric titration was used to determine the amount of iodate, $IO_3^-$, in an unknown
solid. First, 0.8000 gram of the unknown was dissolved in ~50 ml of 0.5 M HCl. Then excess solid KI
was added to it. This solution was then titrated with 0.5600 M $Na_2S_2O_3$. It required 39.60 ml to reach the
end point, which was determined using starch indicator. Calculate the w/w % of $IO_3^-$ in the unknown
solid.
Pertinent reactions:
The FW of $IO_3^-$ = 174.903 g/mole.
$IO_3^-(aq) + 8I^-(aq) + 6H^+(aq) \rightarrow 3I_3^-(aq) + 3H_2O(l)$
$I_3^-(aq) + 2S_2O_3^{2-}(aq) \rightarrow 3I^-(aq) + S_4O_6^{2-}(aq)$
21.(8 pts) The concentration of copper in an unknown may be determined by measuring $E_{cell}$ for the
following electrochemical cell.
SCE || $Cu^{2+}(aq) | Cu^0(s)$
If the cell potential, $E_{cell}$, is measured to be 0.0555 V at a temperature of 298.15 K, what is the
concentration of the $Cu^{2+}$ in the cathodic half cell?
Standard Reduction Potentials:
SCE $E^0$ = 0.241 V
$Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$ $E^0$ = 0.339 V
Express your answer in molarity.
