NJC Prelim P4 Solutions

1 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn over 2018 SH2 H2 Chemistry P4 Solutions Answer all the questions in the spaces provided. 1 Determination of the average relative formula mass of a mixture of two copper salts In this experiment, you will determine the aver age relative formula mass of a mixture of two copper salts by titration. A solution of the copper salts mixture reacts with excess acidified potassium iodide, producing iodine. 2Cu2+(aq) + 4I–(aq) 2Cu I(s) + I2(aq) This iodine is then titrated with aqueous sodium thiosulfate, using starch indicator. FA 1 is an aqueous solution of the copper salt prepared by dissolving 26.0  g of the salt mixture to make 1.00 dm3 of solution. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is aqueous potassium iodide, KI. FA 4 is 1.50 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator (a) Preparation of diluted FA 4 1. Pipette 25.0  cm3 of FA 4 into the 250 cm3 graduated flask. 2. Make up the contents of the flask to the 250 cm 3 mark with deionised water. 3. Stopper the flask and mix the contents thoroughly to ensure a homogeneous solution. This prepared solution is diluted FA 4. Titration 1. Fill the burette with diluted FA 4. 2. Pipette 25.0  cm 3 of FA 1 into a conical flask. 3. Use the measuring cylinder to add approximately 10.0  cm3 of FA 2 to the same conical flask. 4. Use the measuring cylinder to add approximately 20.0  cm3 of FA 3 to the mixture in the conical flask. The mixture will appear brown, due to iodine produced in the reaction. 5. Begin your rough titration by adding diluted FA 4 from the burette until the intensity of the brown colour decreases. 6. Add 10 drops of starch indicator. The mixture will become darker. 7. Continue titrating until the dark colour is discharged. The mixture should appear off-white. This is the end-point. 8. Add one drop of starch indicator to check that no traces of dark colour are produced. 9. If the mixture stays off-white, the titration is completed. If some dark colour is produced, because iodine is still present, continue the titration until mixture appears off-white. 10. Record your burette readings and the rough titre in the space below.

2 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn Over 11. Carry out as many accurate titrat ions as you think necessary to obtain consistent results. 12. Make sure any recorded results show the precision of your practical work. 13. Record in a suitable form below all of your burette readings and the volume of diluted FA 4 added in each accurate titration. I All the following data is recorded initial and final burette readings for two (or more) titrations II Titre values recorded for accurate titrations, and Appropriate headings and units • initial / start (burette) reading • final / end (burette) reading • titre or volume used / added (not “difference”) unit: / cm3 (for each heading) or cm3 unit given for each volume recorded. III: All burette readings are recorded to the nearest 0.05 cm3. (2 d.p) (Rough reading do not need 2 d.p) including 0.00 cm3 (if this was the initial reading). Do not award if: (reverse burette readings) • 50(.00) is used as an initial burette reading • more than one final burette reading is 50.(00) any burette reading is greater than 50.(00) IV: at least two titres within 0.10 cm3 [7] I II III IV V VI VII From your accurate titration results, obtain a suitable value for the volume of diluted FA 4 to be used in your calculations. Show clearly how you obtained this value. Candidate calculates the mean correctly. • Candidate must take the average of two (or more) titres within 0.10 cm 3. • Working / explanation or ticks must be shown • The mean should be quoted to 2 d.p, and be rounded to nearest 0.01 cm3. (e.g. 26.667 cm3 must be rounded to 26.67 cm3) The iodine produced required ............................. cm 3 of diluted FA 4. [1]

3 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn over (b) 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 thiosulfate, Na 2S2O3, in the volume of diluted FA 4 obtained in (a). No of moles of thiosulfate used = 1.50 × ଶହ.଴ ଶହ଴ × mean titre / 1000 Or No of moles of thiosulfate used = 0. 150 × mean titre / 1000 (expressed to 3 or 4 sig fig) moles of Na2S2O3 = ............................. mol [1] (ii) Balance the ionic equation for the reaction of iodine with sodium thiosulfate. State symbols are not required. …..... I 2 + ........ S2O32– → ........ S4O62– + ……... I– Hence calculate the number of moles of iodine that reacted with the number of moles of Na 2S2O3 calculated in (i). Equation balanced I2 + 2 S2O32– → S4O62– + 2I– no of moles of I2 = 0.5 × ans. in (i) moles of I2 = ............................. mol [1] (iii) Using your answer to (ii), calculate the number of moles of copper( II) ions in 25.0 cm3 of FA 1. No of moles of copper(II) ions = 2 × answer (ii) (expressed to 3 or 4 sig fig) moles of Cu 2+ ions = ............................. mol [1] (iv) Using your answer to (iii) and the information on page 2, calculate the average relative formula mass of the copper salts in FA 1. Average Mr = [26.0 / ans (iii)] × 25 / 1000

4 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn Over Average Mr of copper salts = ............................. [1] (v) Write the full electronic configuration of 29Cu in CuI. 1s22s22p63s23p63d10 (vi) Hence, explain why solid CuI appears white in colour. • d subshell is fully filled, no vacant / half-filled d orbitals • electrons cannot be promoted from lower energy d orbitals to higher energy d orbitals / d-d transition not possible • No wavelength of visible spectrum is absorbed, hence solid appears white [1] [Total: 14]

5 NJC SH2 Prelim Exam 9729 / 04/ 18 [Turn over 2 Qualitative analysis In this question, you will deduce the two anions present in FA 1. Perform the tests described in Table 2 and record your observations in the table. Test and identify any gases evolved. If any solution is warmed, a boiling tube MUST be used. Table 2 Test Observations (a) To a 1 cm depth of FA 1 in a test- tube, add aqueous silver nitrate. Solution remained blue / no ppt (b) To a 0.5 cm depth of FA 1 in a boiling tube, add aqueous sodium hydroxide and add one piece of aluminium foil and warm. Blue ppt formed, insoluble in excess sodium hydroxide Black ppt formed upon heating Colourless, pungent gas / gas evolved turns (damp red) litmus blue (c) To a 1 cm depth of FA 1 in a test- tube, add aqueous barium chloride followed by nitric acid. White ppt formed and insoluble in excess nitric acid (d) To a 1 cm depth of FA 1 in a test- tube, add an equal volume of sulfuric acid followed by KMnO4. FA 1 remained blue. Purple KMnO4 remained/ not decolourised [3] (e) From your observations, state the anions present in FA 1. Explain your answers. Nitrate NO3− + Sulfate SO42− Explanation for NO3- Both Nitrate or Nitrite is reduced by Al foil in NaOH(aq) to give NH3 gas + Since KMnO4 remained purple, FA 1 does not contain reducing agent NO2-. Or Since no brown NO 2 gas is evolved with nitric acid, FA 1 contains Nitrate NO3− Explanation for SO 42– Since a white ppt is formed with BaCl2, and remained in nitric acid, anion in FA1 cannot be SO32-. Since Since KMnO4 remained purple, FA 1 does not contain reducing agent SO32-. [3] [Total: 6]

6 NJC SH2 Prelim