USING A REDOX REACTION TO DETERMINE A SOLUTION’S CONCENTRATION The concentration of an aqueous I3- solution can be determined by titration with aqueous sodium thiosulfate, Na2S2O3, in the presence of a starch indicator, which turns from deep blue to colorless when all the I3- has reacted. What is

Question

USING A REDOX REACTION TO DETERMINE A SOLUTION’S CONCENTRATION

The concentration of an aqueous [latex]I_{3}^{-}[/latex] solution can be determined by titration with aqueous sodium thiosulfate, [latex]Na_{2}S_{2}O_{3}[/latex], in the presence of a starch indicator, which turns from deep blue to colorless when all the [latex]I_{3}^{-}[/latex] has reacted. What is the molar concentration of [latex]I_{3}^{-}[/latex] in an aqueous solution if 24.55 mL of 0.102 M [latex]Na_{2}S_{2}O_{3}[/latex] is needed for complete reaction with 10.00 mL of the [latex]I_{3}^{-}[/latex] solution? The net ionic equation is

[latex]2 S_{2}O_{3}^{2-}(aq) + I_{3}^{-}(aq) → S_{4}O_{6}^{2-}(aq) + 3 I^{-}(aq)[/latex]

Accepted Answer

The procedure is similar to that outlined in Figure 4.5. We first need to find the number of moles of thiosulfate ion used for the titration:

[latex]24.55 \cancel{mL} × \frac{1 \cancel{L}}{1000 \cancel{mL}} × \frac{0.102 mol S_{2}O_{3}^{2-}}{1 \cancel{L}} = 2.50 × 10^{-3} mol S_{2}O_{3}^{2-}[/latex]

According to the balanced equation, 2 mol of [latex]S_{2}O_{3}^{2-}[/latex] ion react with 1 mol of [latex]I_{3}^{-}[/latex] ion.
Thus, we can find the number of moles of [latex]I_{3}^{-}[/latex] ion:

[latex]2.50 × 10^{-3} \cancel{mol S_{2}O_{3}^{2-}} × \frac{1 mol I_{3}^{-}}{2 \cancel{mol S_{2}O_{3}^{2-}}} = 1.25 × 10^{-3} mol I_{3}^{-}[/latex]

Knowing both the number of moles of mol) and the volume of [latex]I_{3}^{-} (2.25 × 10^{-3} mol)[/latex] the [latex]I_{3}^{-}[/latex] solution (10.00 mL) then lets us calculate molarity:

[latex]\frac{1.25 × 10^{-3} mol I_{3}^{-}}{10.00 \cancel{mL}} × \frac{10^{3} mL}{1 L}[/latex] = 0.125 M

The molarity of the [latex]I_{3}^{-}[/latex] solution is 0.125 M.

BALLPARK CHECK
According to the balanced equation, the amount of [latex]S_{2}O_{3}^{2-}[/latex] needed for the reaction (2 mol) is twice the amount of [latex]I_{3}^{-}[/latex] (1 mol). The titration results indicate that the volume of the [latex]S_{2}O_{3}^{2-}[/latex] solution (24.55 mL) is a little over twice the volume of the [latex]I_{3}^{-}[/latex] solution (10.00 mL). Thus, the concentrations of the two solutions must be about the sameapproximately 0.1 M.

The reddish [latex]I_{3}^{-}[/latex] solution turns a deep blue color when it is added to a solution containing a small amount of starch.

Question 4.13: USING A REDOX REACTION TO DETERMINE A SOLUTION’S CONCENTRATI...

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