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 /