Task 5.1b

Q1. Find the concentration of an iron II sulfate solution, given that 25.0cm³ of the solution, when acidified required 19.8cm³ of 0.0200 moldm^-3 potassium managanate (VII) for oxidation.
MnO₄^- + 5 Fe²⁺ + 8H⁺ ---> 5 Fe³⁺ + Mn²⁺ + 4H₂O

Q2. Calculate the concentration of a sodium ethanedioate (Na₂C₂O₄) solution, 5.00 cm³ of which were oxidized in acid solution by 24.5 cm³ of a potassium manganate (VII) solution containing 0.05 mol dm^-3.
2MnO₄^- + 16H⁺ + 5C₂O₄^2- ---> 2Mn²⁺ + 8H₂O + 10CO₂

Q3. In an experiment to find the concentration of a solution of iodine, 25.0cm³ of the solution was titrated with 0.100moldm^-3 sodium thiosulfate solution.  18.8cm³ of sodium thiosulfate solution was required to reach the end-point.  Find the concentration of the iodine solution in gdm^-3. (I=127) I₂ +2Na₂S₂O₃ --> 2NaI + Na₂S₄O₆

Q4. 25.0cm³ of a solution of potassium iodate(V) was added to excess potassium iodide solution.  Calculate the concentration of the iodate(V) solution if the iodine librated reacted exactly with 20.0cm³ of 0.100moldm^-3 sodium thiosulfate solution.
IO₃^- + 5I^-  + 6H⁺---> 3I₂ + 3H₂O
2S₂O₃^2- + I₂ = 2I^- + S₄O₆^2-

Q5. A standard solution is prepared by dissolving 1.19g of potassium dichromate(VI) and making up to 250cm³ of solution.  This solution is used to find the  concentration of a sodium thiosulfate solution.  A 25.0cm³ portion of the oxidant was acidified and added to an excess of potassium iodide to liberate iodine.  When the solution was titrated against sodium thiosulfate solution, 17.5cm³ of thiosulfate were required.  Find the concentration of the thiosulfate solution.
Cr₂O₇^2-(aq) + 6I^-(aq) +14H⁺(aq) --> 3I₂(aq) 2Cr³⁺(aq) + 7H₂O(l)

Q6. 25.0cm3 of a solution of potassium manganate(VII) was added to acidified potassium iodide solution.  The iodine liberated reacted with 26.4cm3 of 0.150M sodium thiosulfate solution.  Calculate the concentartion of the potassium managanate(VII) solution
2MnO₄^- + 10I^- + 16H⁺ ---> 2 Mn²⁺ + 8H₂O + 5I₂

A1. amount of  MnO₄^- = vol of MnO₄^- /conc of MnO₄^-
                                   = 19.8/1000 dm³ / 0.0200 moldm^-3 =3.96 * 10^-4 mol
from equation           amount of Fe²⁺/ amount of MnO₄^- = 5/1
amount of Fe²⁺ = 5/1 * amount of  MnO₄^- = 5 * 0.396 mol = 1.98 * 10^-3 mol
conc of Fe²⁺ = amount of Fe²⁺ /volume of Fe²⁺  = 1.98 * 10^-3 mol/ 25.0/1000 dm³ =7.92*10-3 moldm^-3
Concentration of iron II sulfate = conc of Fe²⁺ = 7.92*10^-3 moldm^-3

A5. Concentration of dichromate = 0.0161moldm^-3
amount of dichromate = conc of dichromate * volume of dichromate
                                  = 0.0161moldm^-3 * 25.0*10^-3 dm³ = 4.02*10^-3mol
amount of I₂/amount of Cr₂O₇^2- = 3/1
amount of I₂ = 1.2*10^-3 mol
amount of thiosulfate in 17.5cm³/amount of I₂ =2/1
amount of thiosulfate in 17.5cm³ = 2.42*10^-3mol
conc of thiosulfate = amount of thiosulfate/volume of thiosulfate =0.138moldm^-3