RI 9729/04 2020 Planning

N2020 A Level Planning Question – Suggested Solutions 4(a) n(benzene) = n(C6H5NO2) = 10.0 123 = 0.08180 mol Mass of benzene = 0.08180 x 78.0 = 6.34 g As the yield is 70%, mass of benzene = 6.34 0.70 = 9.06 g 4(b) Volume of benzene required = 9.06 / 0.8765 = 10.34 cm3 We use 10.4 cm3 of benzene for this reaction. Modifying the quantities of reagents based on given procedure which used 8.0 cm3 of benzene. • Volume of concentrated nitric acid = volume of concentrated sulfuric acid = (10.4/8.0) x 8.0 = 10.4 cm3 • Volume of DI water and Na2CO3 for washing organic layer = (10.4/8.0) x 75 = 97.5 ≈ 100 cm3 (I) Preparation of nitrobenzene 1. Using separate 25.0 cm3 measuring cylinders, 10.3 cm3 of conc HNO3 and 10.3 cm3 of conc H2SO4 are added to a clean and dry 100 cm3 conical flask placed in a water bath. 2. Using a 10.0 cm3 measuring cylinder, 2.1 cm3 of benzene is added to the mixture in the conical flask. The mixture is stirred with a thermometer. Care is taken to ensure that the temperature of the reaction mixture does not rise above 50 oC, for example by adding ice to the water bath to cool down the mixture if necessary. 3. Step 2 is repeated till a total of 10.4 cm3 of benzene is added to the reaction mixture. 4. The temperature of the water bath is then adjusted to 60 oC and the reaction mixture is kept at 60 oC for 30 minutes. (II) Purification of nitrobenzene 1. Using a 100 cm 3 measuring cylinder, 100 cm 3 of deionised water is added to a 250 cm 3 separating funnel. The impure nitrobenzene from part (I) is added and the resultant mixture is shaken thoroughly after stoppering the separating funnel. The bottom oily layer is drained into a clean and dry 100 cm 3 beaker from the tap of the separating funnel while the aqueous layer is poured out from the top of the separating funnel. 2. The oily layer in the beaker is poured back into the separating funnel. Using a 100 cm 3 measuring cylinder, 100 cm 3 of aqueous Na 2CO3 is added and the separating funnel shaken thoroughly. The bottom oily layer is drained from the tap while the aqueous layer is poured out from the top of the separating funnel. 3. Step 1 is repeated with another portion of 100 cm3 of deionised water. 4. A spatula of calcium chloride is added to the oily layer in the beaker. The mixture is stirred and left aside for 20 min. 5. The mixture is filtered using clean and dry filter funnel and paper into a 100 cm3 round bottomed flask. 6. A distillation set up is assembled with the round bottomed flask being warmed using a water bath. At 80.1 oC, any remaining benzene will be distilled off. 7. When no more distillate is formed, benzene present as an impurity has been removed. The residue in the round bottomed flask contains the purified nitrobenzene. 4(c) Aqueous sodium carbonate is added to react with excess HNO3 and H2SO4 present. The ionic salts formed from the reaction will dissolve in the aqueous layer and so will be separated