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++7H2OA 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.
79 exam-style questions on AQA A Level Chemistry Amount of substance, covering Relative atomic mass and relative molecular mass, The mole and the Avogadro constant, The ideal gas equation, Empirical and molecular formula, and Balanced equations and associated calculations. Each one has a worked solution and a mark scheme showing where the marks go.