If it takes 35.0 mL of 0.250 M Fe2 + to titrate 100 mL of a solution containing Cr2O72-, what is the molar concentration of Cr2O72-?

0.0146 M

Explanation:

The relevant chemical reaction equation is as follows:

Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ ⇒ 2Cr³⁺ + 6Fe³⁺ + 7H₂O

In order to find the concentration of Cr₂O₇²⁻, the number of moles of Cr₂O₇²⁻ in the solution must be found. This can be found by relating it to the number of moles of the analyte (Fe²⁺), which can be calculated as follows:

(35.0 mL) (0.250mol/L) = 8.75 mmol Fe²⁺

This is the amount of Fe²⁺ required to react with all of the Cr₂O₇²⁻ present. The moles of Fe²⁺ and Cr₂O₇²⁻ in the reaction are related as follows:

8.75 mmol Fe²⁺ x (Cr₂O₇²⁻ / 6Fe²⁺) = 1.4583 ... mmol Cr₂O₇²⁻

The concentration of the Cr₂O₇²⁻ solution is then calculated as follows:

(1.4583 ... mmol) / 100mL = 0.146 mol/L = 0.146 M