RI 2018 Y5 Promo Sections B and C Suggested Solutions

1 2018 Y5 Promotional Examinations Suggested Solutions for Sections B & C B1(a) Kc = [𝐹𝑒3+] [𝐹𝑒2+][𝐴𝑔+] B1(b)(i) Assume the concentration of Ag+ = concentration of SCN─ = 0.045 mol dm-3, or Assume 25.00 cm3 SCN─(aq) was required for titration, (Assume a titre volume between 20.00 cm3 – 25.00 cm3) Amount of Ag+ = 25 1000 x 0.045 = 0.001125 mol Amount of SCN─ in 25.00 cm3 = 0.001125 mol Amount of SCN─ in 250 cm3 = 0.001125 x 10 = 0.01125 mol Mass of KSCN(s) = 0.01125 x (39.1 + 32.1 + 12.0 + 14.0) = 1.0935 = 1.09 g Examiner’s Comments  Many students did not explicitly state the key assumption required to calculate the suitable mass of KSCN to be used.  After calculating the amount of SCN - in the titre volume assumed, many students did not scale up to find the amount of SCN – in 250 cm3 of solution in the volumetric flask. This is stated as a requirement in the question – “You are to prepare a suitable concentration of KSCN solution in a 250cm3 volumetric flask that is…” Please learn to read the question properly and answer the question!  Note that the amount of Fe3+ at equilibrium is irrelevant in this calculation. SCN– reacts with all the Ag+ present. Once all the Ag+ is consumed, the excess drop of SCN- added with reacts with Fe3+, changing the colour of the solution, indicating the end point.  Also, since the solution in the conical flask being titrated does not contain any Ag solid, there is no shifting of the position of equilibrium when the titration is carried out. B1(b)(ii) 1. Using an electronic mass balance, weigh accurately about 1.09 g of KSCN(s) in a dry and clean weighing bottle. 2. Quantitatively transfer KSCN(s) from the weighing bottle to a 100 cm3 beaker. Rinse the weighing bottle a few times with small volumes of deionised water and transfer all washings into the beaker. 3. Dissolve the KSCN(s) in the 100 cm3 beaker. 4. Transfer the solution from the beaker quantitatively into a 250 cm 3 volumetric flask with the aid of a funnel and glass rod. 5. Make up to the 250 cm 3 mark using more deionised water, use a dropper when nearing the mark. 6. Stopper the volumetric flask and shake the volumetric flask well to ensure a homogeneous solution is obtained. Examiner’s Comments  You are advised to write your procedure in clearly numbered steps.  Quantitative transfer This document is copyrighted, please do not reproduce it without permission

2  It is the important that quantitiative transfer of KSCN(s) occurs from the weighing bottle to the beaker and from beaker to volumetric flask. This ensures that the entire mass of KSCN is dissolved in the volumetric flask.  A handful of students described the quantitative transfer process poorly. The ideas of “transferring” and “rinsing” sho