HCI Prelim P4 QP with ans MS

This document consists of 17 printed pages and 1 blank page. Answer all the questions in the spaces provided. 1 Determination of concentrations of sodium hydroxide, and of sodium carbonate in a mixture FA 1 is a solution containing sodium hydroxide, NaOH, and sodium carbonate, Na 2CO3. FA 2 is an aqueous solution containing 0.90 mol dm–3 of hydrochloric acid, HCl. Sodium hydroxide reacts with hydrochloric acid according to the equation below: Reaction 1 NaOH + H Cl  NaCl + H 2O Sodium carbonate reacts with hydrochloric acid in two separate stages. The reactions that occur are: Reaction 2 Na 2CO3 + HCl  NaCl + NaHCO3 Reaction 3 NaHCO 3 + HCl  NaCl + CO2 + H2O You are required to find the concentrations of sodium hydroxide, and of sodium carbonate, in FA 1, by means of a double-indicator titration. In a double-indicator titration, two different indicators are used, separately, in the same titration. In this experiment, thymolphthalein indicator, followed by methyl orange indicator, will be used. Thymolphthalein indicates the end-point when Reaction 1 and 2 are complete, while methyl orange indicates the end-point when Reaction 3 is complete. (a) (i) Dilution of FA 2 Using a burette, measure between 35.00 cm 3 and 36.00 cm 3 of FA 2 into the 250 cm 3 volumetric flask. Record your burette readings and the volume of FA 2 added to the flask in the space below. Final (burette) reading /cm 3 46.30 Initial (burette) reading / cm3 11.00 Volume of FA2 (used for dilution) /cm3 35.30 Make up the contents of the flask to the 250 cm 3 mark with deionised water. Place the stopper in the flask and mix the contents thoroughly by slowly inverting the flask a number of times. This solution is FA 3.

2 2017 HCI C2 H2 Chemistry Preliminary Exam / Paper 4 (ii) Titration of FA 1 against FA 3 Fill a second burette with FA 3. Pipette 25.0 cm 3 of FA 1 into a conical flask. Replace the cap over the FA 1 bottle to prevent absorption of carbon dioxide from the atmosphere. Add a few drops of thymolphthalein indicator and titrate FA 1 with FA 3. The end-point is reached when the solution turns colourless. Ignore any cloudiness that you may observe in the conical flask. Record your titration results in the space below. The volume of FA 3 used to reach the first end-point need not be consistent. Do not discard this solution. To this solution, add a few drops of methyl orange indicator and continue to titrate with FA 3 until the second end-point is reached. Record your titration results in the space below. Perform sufficient titrations to obtain accurate results for the second end-point, which refers to the total volume of FA 3 required for the whole titration. Make certain that all your recorded results show the precision of your working. Initial (burette) reading /cm 3 4.90 4.30 Final (burette) reading 1 /cm3 24.60 24.10 Final (burette) reading 2 / cm3 35.50 34.90 Titre 1 /cm3 19.70 19.80 Titre 2 /cm3 30.60 30.60 Tables have correct headers and units [1] All readings recorded to correct precision [1] Dilutes between 35.00 cm 3 and 36.00 cm3 of FA 2. [1] [3] (b) From your titrations, obtain suitable volumes of FA 3 for the:  first end-point  second end-point. Show clearly how you obtained these volumes. mean titre 1 = ½ (19.70 + 19.80) = 19.75 cm 3 mean titre 2 = ½ (30.60 + 30.60) = 30.60 cm3 Correct average titre from values within 0.10 cm 3 [1] Accuracy (difference between teacher’s and student’s scaled mean titre) [2] volume of FA 3 for first end-point = ........................................................... volume of FA 3 for second end-point = ........................................................... [3]

3 2017 HCI C2 H2 Chemistry Preliminary Exam / Paper 4 (c) (i) Calculate the concentration, in mol dm–3, of HCl in FA 3. [HCl] in FA3 = 35.30 / 250 × 0.90 = 0.127 mol dm–3 concentration of HCl in FA 3 = ........................................................... [1] (ii) Calculate the amount of sodium carbonate, Na2CO3, present in 25.0 cm3 of FA 1. vol. FA3 reacted with NaHCO 3 formed = 30.60 – 19.75 = 10.85 cm 3 Na2CO3 in 25 cm3 FA 1 = NaHCO3 formed = HCl reacted = 0.1271 × 10.85/1000 = 0.00138 mol amount of Na 2CO3 in 25.0 cm3 of FA 1 = .......................................................... [1] (iii) Calculate the amount of sodium hydroxide, NaOH, present in 25.0 cm3 of FA 1. vol. FA3 reacted with NaOH = 19.75 – 10.85 = 8.90 cm 3 [1] NaOH in 25 cm3 FA 1 = HCl reacted = 0.1271 × 8.90/1000 = 0.00113 mol [1] amount of NaOH in 25.0 cm 3 of FA 1 = .......................................................... [2] (iv) Use your answers from (c)(ii) and (c)(iii), calculate the concentrations, in mol dm –3, of Na2CO3 and NaOH in FA 1. [Na 2CO3] in FA 1 = 0.001379 x 1000/25 = 0.0552 mol dm–3 [NaOH] in FA 1 = 0.001131 x 1000/25 = 0.0452 mol dm –3 concentration of Na 2CO3 in FA 1 = .......................................................... concentration of NaOH in FA 1 = .......................................................... [1]

4 2017 HCI C2 H2 Chemistry Preliminary Exam / Paper 4 (d) The maximum error in a single burette reading is ±0.05 cm3. When making up the diluted acid, FA 3, a student recorded that 35.00 cm3 of FA 2 was used. What are the smallest and largest possible volumes of acid that were run into the volumetric flask? smallest volume used = 34.90 cm 3 largest volume used = 35.10 cm3 [1] (e) A student suggested doing the titration in (a)(ii) differently – FA 3 is placed in the conical flask and FA 1 in the burette, using methyl orange indicator followed by thymolphthalein indicator. Explain if this method will allow you to determine the concentrations of NaOH and Na 2CO3 in FA 1. As there is excess HCl in the conical flask, methyl orange only changes colour when both Na 2CO3 and NaOH have completely reacted i.e. only one end-point will be obtained. Hence this method does not allow [Na2CO3] and [NaOH] to be determined. ................................................................................................................................................... ................................................................................................................................................... ...............................................................................................................................................[1] [Total: 13]

5 2017 HCI C2 H2 Chemistry Preliminary Exam / Paper 4 2 Heat of reaction between copper( II) ions and iron metal In this experiment, you will measure the heat given out by the reaction of excess iron with copper(II) sulfate solution and use this to find the concentration of the copper(II) sulfate. Fe(s) + CuSO4(aq)  FeSO4(aq) + Cu(s) FA 4 is iron powder. FA 5 is aqueous copper(II) sulfate, CuSO 4. In an appropriate format in the space below, prepare a table in which you may record each temperature and the time it was taken. 1. Wash thoroughly a burette and fill it with FA 5. Place a Styrofoam cup into a 250 cm 3 beaker to prevent it from tipping over. Transfer 40.00 cm3 of FA 5 into the Styrofoam cup. 2. Place the lid onto the cup and insert the thermometer through the lid. Measure and record the initial temperature of the FA 5 solution in the cup. 3. Start the stopwatch. Measure and record the temperature of the solution in the cup every half minute up to and including the temperature at 1.5 min. Stir the solution using the thermometer. 4. At time t = 2.0 min, add all the powdered iron FA 4 to the solution. Stir the mixture thoroughly with the thermometer to ensure the solid mixes well with the solu