NJC H2 Chem 2013 Prelim P2 Soln

NJC SH2 Prelim 2013 H2 Chemistry Paper 2 1 SH2 H2 Prelim H2 2013 Answers Paper 2 1(a) (i) Ca(IO3)2 (s) Ca2+ (aq) + 2IO3 ‒ (aq) ΔHsol > 0 As temperature is increased, solubility of calcium iodate(V) increases. When temperature is increased, equilibrium shifts right to favour the forward endothermic reaction to partially absorb the extra heat. Hence more Ca(IO3)2 dissolved in water. (ii) [1] 1(b) (i) 2S2O3 2- + I2  2I‒ + S4O6 2- Titrate the iodine produced with thiosulfate solution. (ii) Preliminary calculation Volume of Ca(IO 3)2 used for reaction = 25.0 cm3 (pipette capacity) No. of moles of IO 3 ‒ in the 25.0 cm3 solution = 1 .390 6 . 4 x 2 x 1000 25 = 5.896 x 10-4 mol IO3 ‒ : I 2 : S 2O3 2- 1 : 3 : 6 124

NJC SH2 Prelim 2013 H2 Chemistry Paper 2 2 No. of moles of S 2O3 2- required for titration = 6 x 5.896 x 10 -4 = 3.538 x 10 -3 mol Vol. of S 2O3 2- required for titration = 3.538 cm 3 Volume is too small and will results in high experimental error. Therefore, concentration of FA3 is too high and dilution is required. Volume of H + required for titration = 3.538 cm 3 (no. of moles of H + required is same as S 2O3 2- and their concentrations are the same) Volume of I - required for titration will be less than volume of H + required. Since the I- and H+ can be in excess, 5 cm3 each of FA2 and FA4 are used. Dilution Desired amount of S 2O3 2- from the burette = 25 cm3 Dilution factor = 538. 3 25 Volume of FA 3 to be diluted in a 250 cm 3 volumetric flask = 250 ÷ 538. 3 25 = 35.38 cm 3 1. Using a burette, transfer 35.40cm 3 of FA3 into a clean 250 cm 3 volumetric flask. 2. Top up to the mark with deionized water, stopper, invert and shake to ensure homogeneous solution. 3. Label this solution FA5. Procedure 1. Filter the suspension, FA1, into a clean and dry conical flask using a dry filter paper and funnel. 2. Pipette 25.0 cm 3 of the filtrate into a 250 cm3 conical flask. 3. Using 2 separate 10 cm 3 measuring cylinders, add 5 cm 3 each of FA2 and FA4 into the same conical flask and swirl to ensure mixing. 4. Titrate the contents of the conical flask against FA5 from a burette till a colour change from brown to pale yellow is observed. 5. Add 3 drops of starch indicator and continue titration till the blue-black colour is discharged. 6. Record the volume of FA5 required for titration. 7. Repeat steps 2 to 6 till consistent titration results of within 0.10 cm 3 are obtained. 125

NJC SH2 Prelim 2013 H2 Chemistry Paper 2 3 1(c) experimental data, x = volume of S2O3 2- required for titration (dm3) n(S 2O3 2-) required for titration = (x) × concentration of S2O3 2- = a [IO3 -] = )10 25( 6 3 a = b mol dm-3 [Ca 2+] = ½ b mol dm -3 Ksp = [Ca 2+] [IO3 -