RI H2 Chemistry 2016 Paper 2 TYS Solutions

© Raffles Institution 2016 A level Paper 2 Suggested Solutions Q1 Planning (a) Effervescence would be seen; colourless gas which forms a white ppt with Ca(OH) 2(aq) would be evolved. The deep blue azurite would dissolve completely to give a blue solution. (b) Calculation of a suitable mass of powdered rock to react with about 75% of the acid in the conical flask Cu3(CO3)2(OH)2  3H2SO4 Amt of H2SO4 in 50.00 cm3 = 1.00 x 50.00 x 103 = 0.0500 mol Amt of azurite that reacts with 75% of H2SO4 = 1 3 x 0.75 x 0.0500 = 0.0125 mol Mass of azurite in 0.0125 mol = 0.0125 x 344.5 = 4.306 g Mass of powered rock to be used = 4.306 0.90 = 4.78 g Dilution and Volume of unreacted sulfuric acid required for titration Amt of excess H2SO4 in reaction mixture = 0.25 x 0.0500 = 0.0125 mol Assume that average volume of NaOH required for titration is 25.00 cm3 Amt of excess H2SO4 used for titration = ½ x 0.100 x 25 x 103 = 0.00125 mol (ie 10% of 0.0125 mol) Hence, 10% of excess H2SO4 is needed for titration with NaOH(aq). So, the final reaction mixture can be diluted to 250 cm3 and 25.0 cm3 (ie 10% of 250 cm3) will be pipetted for titration with NaOH. Procedure 1. Using an electronic balance, weigh out accurately about 4.78 g of the powdered rock in a clean and dry weighing bottle. Record the mass of the weighing bottle and powdered rock. 2. Transfer the powdered rock sample into the 250 cm3 conical flask containing 50.00 cm 3 of sulfuric acid and swirl the contents. Place a glass filter funnel on the mouth of the conical flask to prevent acid spray. 3. Reweigh the emptied weighing bottle and record its mass. 4. When effervescence has ended and all the powdered rock has dissolved, t ransfer the final reaction mixture quantitatively into a 250 cm 3 graduated flask with the aid of a funnel and a glass rod. Rinse the conical flask and the glass filter funnel a few times with small volumes of deionised water each time and transfer all the washings into the graduated flask. 4. Fill the graduated flask to the 250 cm 3 mark with more deionised water. Use a teat pipette (or dropper) to add the deionised water drop by drop when nearing the mark. 5. Stopper the graduated flask and shake the solution thoroughly to ensure that it is homogeneous. Label the solution FA 3. 6. Pipette 25.0 cm3 of FA 3 into a 250 cm3 conical flask. Add 2 drops of phenolphthalein indicator. 7. Fill the burette with the 0 .100 mol dm –3 NaOH(aq) provided. Titrate the solution in the conical flask with the standard dilute NaOH(aq) placed in the burette.

© Raffles Institution 8. Stop the titration when one drop of the NaOH(aq) added changes the colour of the solution in the conical flask from blue to light purple. 9. Repeat the titration until at least two consistent results are obtained, i.e. the two titre volumes do not differ by more than 0.10 cm3. Calculations Let that the average values of two consist