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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.

          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.

Problems Solve Each, Show your work for full credit.
20. (8 pts) An iodometric titration was used to determine the amount of iodate, IO3^-, 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 Na2S2O3. It required 39.60 ml to reach the
end point, which was determined using starch indicator. Calculate the w/w % of $IO3^-$ in the unknown
solid.
Pertinent reactions:
The FW of IO3^- = 174.903 g/mole.
IO3^-(aq) + 8I^-(aq) + 6H^+(aq) → 3I3^-(aq) + 3H2O(l)
I3^-(aq) + 2S2O3^2-(aq) → 3I^-(aq) + S4O6^2-(aq)
21.(8 pts) The concentration of copper in an unknown may be determined by measuring Ecell for the
following electrochemical cell.
SCE || Cu^2+(aq) | Cu^0(s)
If the cell potential, Ecell, 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^- → Cu(s) E^0 = 0.339 V
Express your answer in molarity.

Added by Lisa T.

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