Revision notes Group 7/17 Halogens: titrations - volumetric analysis using halide ions, Advanced Inorganic Chemistry
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Doc Brown's Chemistry Advanced Level Inorganic Chemistry Periodic Table Revision Notes
Part 9. Group 7/17 The Halogens
9.9 Volumetric analysis involving halogens or halide ions
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Volumetric titration analysis questions involving halogens or halide ions e.g. iodine titrated with sodium thiosulfate, chloride ion titrated with silver nitrate solution.
PLEASE NOTE KS4 Science GCSE/IGCSE/O Level GROUP 7 HALOGENS NOTES are on a separate webpage
9.9 Volumetric Analysis – titrations involving halogens or halide ions
(1) Chloride ions can be titrated with standardised silver nitrate solution.
The questions below are from the Volumetric (non–redox) Titration Calculations Page with ANSWERS!
Q10 25 cm3 of seawater was diluted to 250 cm3 in a graduated volumetric flask. A 25 cm3 aliquot of the diluted seawater was pipetted into a conical flask and a few drops of potassium chromate(VI) indicator solution was added.
On titration with 0.1 mol dm–3 silver nitrate solution, 13.8 cm3 was required to precipitate all the chloride ion. [Atomic masses: Na = 23, Cl = 35.5]
Q11 0.12 g of rock salt was dissolved in water and titrated with 0.1 mol dm–3 silver nitrate until the first permanent brown precipitate of silver chromate was seen.
Q12 5.0 g of a solid mixture of anhydrous calcium chloride(CaCl2) and sodium nitrate (NaNO3) was dissolved in 250 cm3 of deionised water in a graduated volumetric flask. A 25 cm3 aliquot of the solution was pipetted into a conical flask and a few drops of potassium chromate(VI) indicator solution was added.
The questions above are from the Volumetric (non–redox) Titration Calculations Page with ANSWERS!
(2) Iodine can be titrated with standardised sodium thiosulfate solution.
Question 2: Given the following two half–reactions
(a) Given (i) S4O62–(aq) + 2e– ==> 2S2O32–(aq)
and (ii) I2(aq) + 2e– ==> 2I–(aq)
construct the full ionic redox equation for the reaction of the thiosulphate ion S2O32–,and iodine.
(b) what mass of iodine reacts with 23.5 cm3 of 0.012 mol dm–3 sodium thiosulphate solution.
(c) 25cm3 of a solution of iodine in potassium iodide solution required 26.5 cm3 of 0.095 mol dm–3 sodium thiosulphate solution to titrate the iodine.
What is the molarity of the iodine solution and the mass of iodine per dm3?
Question 17: 25.0 cm3 of an iodine solution was titrated with 0.1 mol dm–3 sodium thiosulphate solution and the iodine reacted with 17.6 cm3 of the thiosulphate solution.
(a) give the reaction equation.
(b) what indicator is used? and describe the end–point in the titration.
(c) calculate the concentration of the iodine solution in mol dm–3 and g dm–3.
Question 14: Given the half–cell reaction IO3–(aq) + 6H+(aq) + 5e– ==> 1/2I2(aq) + 3H2O(l)(see also Q2)
(a) Deduce the redox equation for iodate(V) ions oxidising iodide ions.
(b) What volume of 0.012 mol dm–3 iodate(V) solution reacts with 20.0 cm3 of 0.100 mol dm–3 iodide solution?
(c) 25.0 cm3 of the potassium iodate solution were added to about 15 cm3 of a 15% solution of potassium iodide (ensures excess iodide ion). On acidification, the liberated iodine needed 24.1 cm3 of 0.05 mol dm–3 sodium thiosulphate solution to titrate it.
(i) Calculate the concentration of potassium iodate(V) in g dm–3
(ii) What indicator is used for this titration and what is the colour change at the end–point?
Question 18: 1.01g of an impure sample of potassium dichromate(VI), K2Cr2O7, was dissolved in dil. sulphuric acid and made up to 250 cm3 in a calibrated volumetric flask. A 25 cm3 aliquot of this solution pipetted into a conical flask and excess potassium iodide solution and starch indicator were added. The liberated iodine was titrated with 0.1 mol dm–3 sodium thiosulphate and the starch turned colourless after 20.0 cm3 was added.
(a) Using the half–equations from Q3(a)(ii) and Q2(a)(ii), construct the full balanced equation for the reaction between the dichromate(VI) ion and the iodide ion.
(b) Using the half–equations from Q2(a) construct the balanced redox equation for the reaction between the thiosulphate ion and iodine.
(c) Calculate the moles of sodium thiosulphate used in the titration and hence the number of moles of iodine titrated.
(d) Calculate the moles of dichromate(VI) ion that reacted to give the iodine titrated in the titration.
(e) Calculate the formula mass of potassium dichromate(VI) and the mass of it in the 25 cm3 aliquot titrated.
(f) Calculate the total mass of potassium dichromate(VI) in the original sample and hence its % purity.
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