RI 2021 Year 6 March TP Suggested Solutions

© Raffles Institution 2020 9729/M/20 2021 Y6 H2 Chemistry Timed Practice – Suggested Solutions Section B B1 (a) 3 3 [NH (aq)] [NH (org)] ≈ 25 ⇒ [NH3(aq)] = 25[NH3(org)] [NH3(aq)] + [NH3(org)] = 1.04 25[NH3(org)] + [NH3(org)] = 1.04 [NH3(org)] = 0.04 mol dm–3 [NH3(aq)] = 1.04 – 0.04 = 1.00 mol dm–3 Amount of NH3 in 25.0 cm3 sample of NH3(aq) = 25.0 1.00 0.0250 mol1000= Since NH3 and HCl react in a 1:1 ratio, Amount of HCl required for titration = 0.0250 mol Volume of HCl = 330.0250 0.250 dm 250 cm0.100 == Volume of HCl is unsuitable because it exceeds the burette capacity of 50.00 cm3, therefore it is tedious to refill the burette multiple times . This is a result of the concentration of ammonia being too high. Examiners’ Comments • Some students did not read the question and failed to realise that there are two parts to the question – 1. To calculate the volume of HCl required; 2. To explain why [NH 3] is unsuitable. • Careful reading of the information provided will lead students to realise that the NH3 is dissolved in both layers. Some students assumed that 1.04 mol dm–3 was the [NH3] in the aqueous layer. • To calculate the volume of HCl required to titrate the aqueous layer, the concentration of NH3 in the aqueous layer should be calculated first. • Students need to elaborate beyond the fact that the volume of HCl required exceeds the burette capacity by explaining, for example, that there is thus a need to refill the burette, causing the experiment to become tedious. (b) Ideally, 25.0 cm3 of diluted ammonia would require 25.0 cm3 of 0.100 mol dm–3 hydrochloric acid. Since 250 cm 3 of hydrochloric acid was required, a sample of the aqueous layer needs to be diluted 10 times i.e. obtain 25.0 cm 3 of aqueous ammonia and dilute to 250 cm3. 1. Drain the bottom organic layer into a 100 cm 3 conical flask / beaker to discard. Pour/drain the remaining aqueous layer into another 100 cm3 conical flask / beaker. 2. Use a 25.0 cm 3 pipette to t ransfer 25.0 cm 3 of the aqueous layer solution into a 250 cm3 volumetric flask. This document is copyrighted, please do not reproduce it without permission

© Raffles Institution 2020 9729/M/20 3. Fill the graduated flask to the 250 cm3 mark with more deionised water. Use a dropping pipette (or teat pipette or dropper) to add the deionised water drop by drop when nearing the mark. 4. Stopper the graduated flask and shake the solution thoroughly to ensure that it is homogeneous. Label the solution FA 1. 5. Pipette 25.0 cm 3 of FA 1 into a 250 cm 3 conical flask. Add 2 drops of methyl orange indicator and swirl the conical flask. 6. Titrate the solution in the conical flask with the standard 0.100 mol dm –3 hydrochloric placed in the burette. 7. Stop the titration when the end -point of the titration is reached i.e. when one drop of the hydrochloric acid added changes the colour of the solution in the conical flask from yellow to orange. 8. Record the titration readings usi