Ammonium iron(II) sulfate hexahydrate is used to find the concentration of solutions of potassium dichromate(VI) by titration. The equation for this reaction is:
Cr2O72−+14H++6Fe2+→2Cr3++6Fe3++7H2O\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{Fe}^{2+} \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}Cr2O72−+14H++6Fe2+→2Cr3++6Fe3++7H2O
A standard solution is made by dissolving 1.470 g1.470\text{ g}1.470 g of (NH4)2Fe(SO4)2⋅6H2O(\text{NH}_4)_2\text{Fe}(\text{SO}_4)_2 \cdot 6\text{H}_2\text{O}(NH4)2Fe(SO4)2⋅6H2O (Mr=392.0M_r = 392.0Mr=392.0) in deionised water acidified with dilute sulfuric acid, and making up to 250.0 cm3250.0\text{ cm}^3250.0 cm3 in a volumetric flask.
25.0 cm325.0\text{ cm}^325.0 cm3 of this solution is pipetted into a conical flask. The mixture is titrated with potassium dichromate(VI) solution. The titration is repeated until concordant results are obtained. The mean titre is 18.45 cm318.45\text{ cm}^318.45 cm3.
Calculate the concentration, in mol dm−3\text{mol dm}^{-3}mol dm−3, of the potassium dichromate(VI) solution. Give your answer to 3 significant figures.