7. The number of of carbon monoxide in a sample of air can be measured as follows. Step 1 The carbon monoxide reacts with iodine(V) oxide, producing iodine. 5CO(g) + I2O5(s) I2(s) + 5CO2(g) → Step 2 The iodine is then dissolved in potassium iodide solution and titrated against sodium thiosulphate solution. I2(aq) + 2S2O S4O + 2I– → aq) (a) Write the ion-electron equation for the oxidation reaction in Step 2. (1) (b) Name a chemical that can be used to indicate when all of the iodine has been removed in the reaction taking place in Step 2. (1) (c) If 50.4cm3 of 0.10 mol l–1 sodium thiosulphate solution was used in a titration, calculate the number of moles of carbon monoxide in the sample of air. Show your working clearly. (2) 8. A major problem for the developed world is the pollution of rivers and streams by nitrite and nitrate ions. The concentration of nitrite ions, NO2 in water –(aq), can be determined by titrating samples against acidified permanganate solution. (a) Suggest two points of good practice that should be followed to ensure that an accurate end-point is achieved in a titration. (b) An average of 21·6cm3 of 0·0150 mol l–1 acidified permanganate solution was required to react completely with the nitrite ions in a 25·0 cm3 sample of river water. The equation for the reaction taking place is: + 2+ – – – 2MnO4 + 5NO2 + 6H 2Mn + 5NO3 + 3H2O(l) → (aq) (aq) (aq) (aq) (aq) (i) Calculate the nitrite ion concentration, in mol l–1, in the river water. Show your working clearly. (2) (ii) During the reaction the nitrite ion is oxidised to the nitrate ion. Complete the ion-electron equation for the oxidation of the nitrite ions. – – NO2 NO3 (1) → (aq) (aq)
7. The number of of carbon monoxide in a sample of air can be measured as follows. Step 1 The carbon monoxide reacts with iodine(V) oxide, producing iodine. 5CO(g) + I2O5(s) → I2(s) + 5CO2(g) Step 2 The iodine is then dissolved in potassium iodide solution and titrated against sodium thiosulphate solution. I2(aq) + 2S2O → S4O + 2I– aq) (a) Write the ion-electron equation for the oxidation reaction in Step 2. (1) (b) Name a chemical that can be used to indicate when all of the iodine has been removed in the reaction taking place in Step 2. (1) (c) If 50.4cm3 of 0.10 mol l–1 sodium thiosulphate solution was used in a titration, calculate the number of moles of carbon monoxide in the sample of air. Show your working clearly. (2) 8. A major problem for the developed world is the pollution of rivers and streams by nitrite and nitrate ions. The concentration of nitrite ions, NO2 –(aq), in water can be determined by titrating samples against acidified permanganate solution. (a) Suggest two points of good practice that should be followed to ensure that an accurate end-point is achieved in a titration. (b) An average of 21·6cm3 of 0·0150 mol l–1 acidified permanganate solution was required to react completely with the nitrite ions in a 25·0 cm3 sample of river water. The equation for the reaction taking place is: – – + 2+ – 2MnO4 (aq) + 5NO2 (aq) + 6H (aq) → 2Mn (aq) + 5NO3 (aq) + 3H2O(l) (i) Calculate the nitrite ion concentration, in mol l–1, in the river water. Show your working clearly. (2) (ii) During the reaction the nitrite ion is oxidised to the nitrate ion. Complete the ion-electron equation for the oxidation of the nitrite ions. – – NO2 (aq) → NO3 (aq) (1)
Unit-3-Exam-Level-Questions-1.docx