Cerium(IV) sulfate, Ce(SO )

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

Cerium(IV) sulfate, Ce(SO

_4

)

_2

, is used as a titrant for determining the concentration of Fe

^{2+}

in aqueous solution. A student writes the following balanced redox equation for the titration that occurs in acidic medium:

\text{Ce}^{4+} + \text{Fe}^{2+} \longrightarrow \text{Ce}^{3+} + \text{Fe}^{3+}

To shorten the calculation, another student incorrectly modifies the equation to

2\,\text{Ce}^{4+} + \text{Fe}^{2+} \longrightarrow 2\,\text{Ce}^{3+} + \text{Fe}^{3+}

and then uses this flawed stoichiometry to determine the molarity of an Fe

^{2+}

solution by titration. Analyze the impact of this error. (a) Verify that the first equation is properly balanced for both atoms and charge. Provide the net charge on each side. (b) Show that the modified equation violates conservation of charge, explicitly calculating the charges on both sides. (c) Suppose 25.00 mL of Fe

^{2+}

solution requires 18.42 mL of 0.0500 M Ce

^{4+}

solution to reach the endpoint. Using the correct balanced equation, calculate the molarity of the Fe

^{2+}

solution. (d) If the student had used the incorrect stoichiometry in part (b), would the reported molarity of Fe

^{2+}

be too high, too low, or unchanged? Quantitatively justify your answer by recalculating the molarity with the flawed equation and comparing the results.

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