CIE Jun 2016 v2 Paper 3 Q2

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You will determine the amount of sulfuric acid remaining in flask X after the reaction with magnesium in Question 1. You will do this by titration with sodium hydroxide of known concentration. 2NaOH(aq) + H2SO4(aq)Na2SO4(aq) + 2H2O(l) PMT FB 3 is 0.150 mol dm3 sodium hydroxide, NaOH. bromophenol blue indicator (a) MethodTransfer all the contents of flask X from Question 1 into the 250 cm3 volumetric flask. Rinse flask X with distilled water and add the washings to the volumetric flask. Add distilled water up to the mark. Stopper the volumetric flask and mix the contents thoroughly. Label this solution FB 4. Rinse the pipette and use it to transfer 25.0 cm3 of FB 4 into the conical flask. Add about 10 drops of bromophenol blue to the conical flask. Fill the burette with FB 3. Perform a rough titration and record your burette readings in the space below. The end point is reached when the solution becomes a permanent blue-violet colour. The rough titre iscm3. Carry out as many accurate titrations as you think necessary to obtain consistent results. Record, in a suitable form below, all of your burette readings and the volume of FB 3 added in each accurate titration. Make certain any recorded results show the precision of your practical work. II III IV VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FB 3 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FB 4 requiredcm3 of FB 3. [1]UCLES 2016 9701/32/M/J/16<br /> PMT (c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of sodium hydroxide, NaOH, in the volume of FB 3 you calculated in (b). (ii) Use your answer to (i) to calculate the number of moles of sulfuric acid present in the 25.0 cm3 of FB 4 pipetted in (a). moles of NaOH =mol (iii) Use your answer to (ii) to calculate the number of moles of sulfuric acid, H2SO4, remaining in flask X after the reaction in 1(a). moles of H2SO4 =mol moles of H2SO4 remaining from 1(a) =mol (iv) Use the relevant information on page 2 to calculate the number of moles of sulfuric acid, H2SO4, pipetted into reaction flask X in 1(a). moles of H2SO4 pipetted into flask X =molUCLES 2016 9701/32/M/J/16 [Turn over<br /> (v) Use your answers to (iii) and (iv) to calculate the number of moles of sulfuric acid which reacted with the magnesium in flask X. PMT moles of H2SO4 which reacted in flask X =mol (vi) Use your answer to (v) and the mass of magnesium used in 1(a) to calculate the relative atomic mass, Ar, of magnesium. Ar of magnesium =[6] (d) (i) A student, who carried out the experiments in Questions 1 and 2 correctly, calculated the Ar of magnesium as shown in the table. Question 1 Question 2 Ar Mg 20.8 22.5 Use the Ar of magnesium given in the Periodic Table on page 12 to deduce which practical procedure is less accurate. Identify one source of inaccuracy and explain one change the student could make in order to improve the accuracy.is less accurate source of inaccuracyimprovement(ii) Use the Ar of magnesium given in the Periodic Table to calculate the percentage error in the students value from Question 1.[3] [Total: 17]UCLES 2016 9701/32/M/J/16<br />

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