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 t