SRJC_2008 JC2 Prelim H2 Paper 2 Solutions

1 SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CHEMISTRY 9746/02 Preliminary Examination Paper 2 Solutions 1 Potassium manganate (VII) is an oxidising agent used in redox titrations. A 50.0 cm 3 sample of iron ( II) oxalate, FeC 2O4, was dissolved in water and the solution made up to 250 cm 3. A 25.0 cm 3 portion of this solution was acidified and titrated with 20.80 cm3 of 0.025 mol dm-3 potassium manganate (VII). (a) (i) State the change in oxidation number for manganese. The oxidation number of Mn decreases from +7 in MnO4 - to +2 in Mn2+. (ii) Write an ionic equation for the oxidation of oxalate ion to carbon dioxide. C 2O4 2- Æ 2CO2 + 2e (iii) Write an overall equation for the reaction between potassium manganate (VII) and iron (II) oxalate. 5Fe 2+ + 5C2O4 2- + 3MnO4 - + 24H+ Æ 5Fe3+ + 10CO2 + 3Mn2+ + 12H2O (iv) Calculate the concentration of iron (II) oxalate in the original sample. 5FeC 2O4 ≡ 3MnO4 - Amount of Fe 2C2O4 in 25.0 cm3 portion = 025 . 01000 80 . 20 3 5 × × = 8.667 x 10-4 mol Amount of Fe 2C2O4 in original sample = 410 667 . 80 . 25 250 −× × = 8.667 x 10 -3 mol Original [Fe 2C2O4] = 100050 10 667 . 83−× = 0.173 mol dm-3 (v) State the colour change at t he endpoint of the titration. Purple to yellow or purple to colourless

2 In the above redox titration, carbon dioxid e gas is produced. One of the uses of carbon dioxide is in the manufacture of carbonated drinks. Cola, a carbonated drink, was originally invented by chemist, John Pemberton, to stop headaches and calm nervousness. Cylinders of pressurised carbon dioxi de are used to produce cola. A commercial cola drink was manufactured using such cylinde rs, each with internal volume of 5 dm 3 and contains 2.58 kg of carbon dioxi de. The pressure inside each of the cylinder was found to be 1.0 x 104 kPa at room temperature. A glass of cola is fizzy because carbon dioxide has been dissolved in it under pressure. When 500 cm 3 of commercial cola drink is poured out from a bottle, the carbon dioxide is gradually released as bubbles of gas . On evolution of 1 dm 3 of carbon dioxide, the cola went flat, as the concentration of dissolved carbon dioxide decreases to its saturation level of 1.5 g dm -3 at room temperature. (b) (i) Using the ideal gas equation pV = nRT, calculate the pressure the carbon dioxide would exert inside the cylinder at room temperature. MV mRTp = 3 3 10 5 0 . 2 44 298 314 . 8 10 −× × × × ×58 . = = 2.91 x 107 Pa (ii) Suggest why the pressure you calculated in b(i) differs from the actual value. CO2 does no