ACJC 2022 H2 Chem Paper 4 QP and Ans

3 © ACJC 2022 9729/04/Prelim/2022 [Turn over Answer all the questions in the spaces provided. 1 Identification of an unknown organic acid by titration In this experiment, you will perform titrations to determine the relative molecular mass, Mr, of an unknown monoprotic organic acid, FA 1, and use it to identify the acid. FA 1 is an aqueous solution containing 4.80 g dm–3 of an unknown monoprotic organic acid. You are also provided with: FA 2 is 0.110 mol dm–3 sodium hydroxide, NaOH. thymol blue indicator Prepare a table in the space provided below to record, to an appropriate level of precision, all your burette readings. (a) Titration of FA 1 with FA 2 1. Fill the burette with FA 2. 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Add 3 to 4 drops of thymol blue indicator into the flask. The solution in the flask should turn pink. 4. Titrate FA 1 in the conical flask with FA 2. The end-point is reached when the solution changes from yellow to green. 5. Record your burette readings in your table. 6. Repeat steps 2 to 5 until consistent results are obtained. Titration Results 1 2 final burette reading / cm3 23.50 47.50 initial burette reading / cm3 0.00 24.00 volume of FA 2 added / cm3 23.50 23.50 [5] From your titrations, obtain a suitable volume of FA 2, to be used in your calculations. Show clearly how you obtained this volume. volume of FA 2 = ……………………….. [1]

4 © ACJC 2022 9729/04/Prelim/2022 [Turn over (b) (i) Calculate the amount of sodium hydroxide present in the volume of FA 2 that you calculated in (a). amount of NaOH reacted = ……………………. [1] (ii) Calculate the concentration of organic acid present, in mol dm–3, in FA 1. . concentration of organic acid in FA 1 = ……………………….. [1] (iii) Calculate the relative molecular mass, Mr, of the organic acid in FA 1. Mr of the organic acid = ……………………….. [1]

5 © ACJC 2022 9729/04/Prelim/2022 [Turn over (iv) From another experiment, the identity of FA 1 was narrowed down to these four acids. CH3COOH HCOOH CH2=CHCO2H CH3CH2CO2H Given that sodium hydroxide reacts only with the COOH group in the organic acid, deduce the identity of the organic acid present in FA 1. Explain your answer. [Ar: H, 1.0; C, 12.0; O, 16.0; Cl, 35.5] ………..……………………………………………………………………….….…….. ………………………………………………………………………….....…………...[1] (c) This identification method uses the calculated relative molecular mass of the acid. As the relative molecular masses of CH 2=CHCO2H and CH 3CH2CO2H are so similar, any slight inaccuracy in the titration could lead to an incorrect conclusion. Describe a chemical test that would enable you to distinguish between CH 2=CHCO2H and CH3CH2CO2H. There is no need to carry out this test. ………..……………………………………………………………….……………….………. ………..……………………………………………………………….……………….………. ………………………………………………………………………………….....………… [2] (d) Another student was provided a solution of another unknown organic acid. It has the same concentration, in mol dm –3, as that used in the titration above. However, this new unknown organic acid is a diprotic acid. State and explain how the calculated relative molecular mass obtained will differ from that obtained in b(iii). ………..……………………………………………………………….……………….………. ………..……………………………………………………………….……………….………. ………..……………………………………………………………….……………….………. ………………………………………………………………………………….....………… [2] [Total: 14]

6 © ACJC 2022 9729/04/Prelim/2022 [Turn over 2 Determination of the concentration of NaOH and the enthalpy change of neutralisation, Hneut You are provided with the following solutions: FA 3 is a solution of sodium hydroxide, NaOH FA 4 is a solution of 2.00 mol dm–3 hydrochloric acid, HCl In this question, you are to perform a series of 6 experiments where different volumes of FA 3 and FA 4 are mixed together to give a total volume of 50 cm 3. The temperature change, T, for each experiment will be determined and a graph of T against the volume of FA 3 will be plotted. You will then use the data from your graph to determine the concentration of NaOH in FA 3, and the enthalpy change of neutralisation for the reaction between aqueous sodium hydroxide and hydrochloric acid. NaOH + HCl → NaCl + H2O Hneut (a) Procedure Experiment 1 1) Place one polystyrene cup inside a second polystyrene cup. Place these into a 250 cm3 beaker to prevent them from tipping over. 2) Use a 50 cm3 measuring cylinder to transfer 10.0 cm3 of FA 3 into the polystyrene cup. 3) Measure the temperature of FA 3 in the polystyrene cup and record the initial temperature of FA 3, T1, in Table 2.1 on page 7. 4) Use another 50 cm3 measuring cylinder, transfer 40.0 cm3 of FA 4 into the same polystyrene cup. 5) Stir the mixture in the polystyrene cup with the thermometer. Measure and record the highest temperature, T2 in Table 2.1 on page 7. 6) Rinse the polystyrene cup and thermometer with distilled water and dry them with paper towels. 7) Repeat steps 2 to 6 using 20.0 cm3, 30.0 cm3 and 40.0 cm3 of FA 3, each time using appropriate volume of FA 4, such that the total volume of the reaction mixture is 50 cm3. 8) Record all measurements of volume, temperature ( T1 and T2) and temperature change, T, in Table 2.1 on page 7.

7 © ACJC 2022 9729/04/Prelim/2022 [Turn over (b) Results Table 2.1 experiment 1 2 3 4 5 6 volume of FA 3 / cm3 10.0 20.0 30.0 40.0 25.0 35.0 volume of FA 4 / cm3 40.0 30.0 20.0 10.0 25.0 15.0 initial temperature, T1/ oC 29.0 29.0 29.0 29.0 29.0 29.0 final temperature, T2/ oC 33.6 38.0 40.0 34.4 40.0 38.0 temperature change, T/ oC +4.6 +9.0 +11.0 +5.4 +11.0 +9.0 [3] (c) (i) Plot a graph of T (y–axis) against volume of FA 3 used (x–axis) using the four experimental results that you have obtained. The scales for both axes must be chosen to provide an origin.

8 © ACJC 2022 9729/04/Prelim/2022 [Turn over By considering your plotted points, perform two additional experiments to identify the volume of FA 3 needed to produce the maximum temperature change, Tmax. In each experiment, ensure that the total volume of the reaction mixture is 50 cm3. You may find it helpful to plot the results from each experiment before choosing the volumes to use in the next experiment. Record all measurements of volume, temperature (T1 and T2) and temperature change, T, in Table 2.1 on page 7. [2] (ii) Draw two straight lines of best fit. The first best fit line should be drawn using the plotted points where T is increasing and the second best fit line should be drawn using the plotted points where T is decreasing. Extrapolate these two lines until they cross. [1]

9 © ACJC 2022 9729/04/Prelim/2022 [Turn over (d) (i) Use your graph to determine the maximum temperature change, Tmax, as well as the volume of FA 3, VFA3, u