A Level Chemistry 9701/22/F/M/16 Q2
2 Spathose is an iron ore that contains iron(II) carbonate, FeCO3. The percentage of iron(II) carbonate in spathose can be determined by titration with acidified potassium dichromate(VI) solution using a suitable indicator.The ionic equation is shown below.
Cr2O7 2–(aq) + 14H+(aq) + 6Fe2+(aq) → 2Cr3+(aq) + 6Fe3+(aq) + 7H2O(l)
(a) A 5.00g sample of spathose was reacted with excess concentrated hydrochloric acid and then filtered. The filtrate was made up to 250cm3 in a volumetric flask with distilled water.
A 25.0cm3 sample of the standard solution required 27.30cm3 of 0.0200moldm–3
dichromate(VI) solution for complete reaction.
(i) Calculate the amount, in moles, of dichromate(VI) ions used in the titration.
(ii) Use your answer to (i) to calculate the amount, in moles, of Fe2+ present in the 25.0cm3 sample.
(iii) Use your answer to (ii) to calculate the amount, in moles, of Fe2+ present in the 250cm3 volumetric flask.
(iv) Use your answer to (iii) to calculate the mass of iron(II) carbonate present in the sample of spathose.
(v) Calculate the percentage of iron(II) carbonate in the sample of spathose.
(b) Iron ores containing iron(III) compounds can be analysed using a similar method. A standard solution of an aqueous iron(III) compound is reacted with aqueous tin(II) chloride. Aqueous tin(IV) chloride and aqueous iron(II) chloride are the products of this reaction.
(i) Write an ionic equation for this reaction. Do not include state symbols.
(ii) Any excess tin(II) chloride can be removed by reaction with HgCl 2(aq). A white precipitate of Hg2Cl 2 is produced. Complete the equation for this reaction.