2024 ACJC H2 Chemistry Prelim P4 (Ans)

2 © ACJC 2024 9729/04/Prelim/2024 [Turn over Answer all the questions in the spaces provided. 1 To determine the concentration of NaOH and Na2CO3 in FA 1 Aqueous NaOH can be used to remove CO 2 from exhaled air in spacecrafts. NaOH reacts with CO2 according to the following equation: 2NaOH(aq) + CO2(g) → Na2CO3(aq) + H2O(l) A sample of aqueous NaOH was used to absorb CO2 in the air for some time. The resulting mixture of NaOH and Na2CO3 formed was labelled as FA 1. You are to determine the concentration of NaOH and Na 2CO3 in FA 1 by titration with an aqueous solution of HCl, FA 2. You are provided with the following. FA 1, is a mixture of NaOH and Na2CO3 FA 2, 2.50 mol dm−3 of hydrochloric acid, HCl methyl orange indicator thymol blue indicator distilled water (a) Titration of FA 1 against FA 2 using thymol blue indicator. 1. Fill a burette with FA 2. 2. Using a pipette, transfer 25.0 cm3 of FA 1 into a 250 cm3 conical flask. 3. Add a few drops of thymol blue indicator into the conical flask. 4. Run FA 2 from the burette into the flask. The end point is reached when the solution changes from blue to green. 5. Record your titration results in Table 1.1 6. Repeat points 2 to 5 until consistent titre values are obtained. Table 1.1 1 2 3 final burette reading / cm3 13.80 13.80 initial burette reading / cm3 0.00 0.00 volume of FA 2 added / cm3 13.80 13.80 [3] Students should be mindful to input the burette reading in the correct box. Students should be careful with the subtraction of the burette readings and all entries must be written to 2 dp (to nearest 0.05 cm3). Students should have at least 2 consistent readings within ±0.10 cm3.

3 © ACJC 2024 9729/04/Prelim/2024 [Turn over 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] Calculation of average to 2 dp (b) Titration of FA 1 against FA 2 using methyl orange indicator. Repeat points 1 to 6 in (a) but add methyl orange at point 3 in place of thymol blue. Using this indicator, the end point is reached when the solution changes from yellow to orange. Record your titration results in Table 1.2. Table 1.2 1 2 3 final burette reading / cm3 18.80 19.00 initial burette reading / cm3 0.00 0.20 volume of FA 2 added / cm3 18.80 18.80 [3] 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] (c) Calculation When thymol blue is used as the indicator in part (a), the following reactions occur. Reaction 1: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Reaction 2: Na2CO3(aq) + HCl(aq) → NaCl(aq) + NaHCO3(aq) Wh