A sample of iron(II) sulfate crystals FeSO47H2O had been left out in the air and some of the Fe2ions had been converted to Fe3. A total of 4.2g of the impure crystals were dissolved in a total of 250cm3 of water and dilute sulfuric acid. Portions of 25cm3of this solution were titrated with a solution of potassium dichromate(VI). The concentration of dichromate(VI) ions in this solution was 0.1moldm3.The average titre was 23.50cm3. Use the following steps to find the percentage purity of the original crystals.