Dichromate ion, Cr2O7 2-, reacts with aqueous iron(II) ion in acidic solution according to the balanced equation What is the concentration of Fe2+ if 46.99 mL of 0.2004 M K2Cr2O7 is needed to titrate 50.00 mL of the Fe2+ solution?

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1. Identify the balanced chemical equation for the reaction: \( \text{Cr}_2\text{O}_7^{2-} + 6\text{Fe}^{2+} + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O} \).
2. Calculate the moles of \( \text{Cr}_2\text{O}_7^{2-} \) using its concentration and volume: \( \text{moles of } \text{Cr}_2\text{O}_7^{2-} = 0.2004 \text{ M} \times 0.04699 \text{ L} \).
3. Use the stoichiometry of the balanced equation to find the moles of \( \text{Fe}^{2+} \): \( 1 \text{ mole of } \text{Cr}_2\text{O}_7^{2-} \) reacts with \( 6 \text{ moles of } \text{Fe}^{2+} \).
4. Calculate the moles of \( \text{Fe}^{2+} \) using the stoichiometric ratio from the balanced equation.
5. Determine the concentration of \( \text{Fe}^{2+} \) by dividing the moles of \( \text{Fe}^{2+} \) by the volume of the \( \text{Fe}^{2+} \) solution in liters: \( \text{Concentration of } \text{Fe}^{2+} = \frac{\text{moles of } \text{Fe}^{2+}}{0.05000 \text{ L}} \).

Key Concepts

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions. It involves using balanced chemical equations to determine the relationships between the amounts of substances consumed and produced. In this case, understanding the stoichiometric ratios between dichromate ions and iron(II) ions is essential for calculating the concentration of Fe2+.

Titration

Titration is a laboratory technique used to determine the concentration of a solute in a solution. It involves the gradual addition of a titrant (in this case, K2Cr2O7) to a solution of unknown concentration (Fe2+) until the reaction reaches completion, indicated by a color change or other measurable endpoint. The volume and concentration of the titrant allow for the calculation of the unknown concentration.

Molarity

Molarity is a measure of concentration defined as the number of moles of solute per liter of solution. It is expressed in moles per liter (M). In this problem, the molarity of K2Cr2O7 is given, and it is crucial for determining how many moles of dichromate are used in the titration, which can then be related to the moles of Fe2+ present in the solution.