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” should be clearly described.  Students are reminded to consider carefully their choice of words. For example, “fill up the volumetric flask with deionised water to the 250cm 3 mark” does not mean the same meaning as “fill up the volumetric flask with deionised water”. Please revise and be familiar with common terms used to describe these processes.  Some common mistakes made are highlighted in the following table and explanations of why these should not be done are included. Please take note of these and do not repeat these mistakes. Common mistake Why this should not be done What should be done Many students take a shortcut by directly transfer the solid from weighing bottle to volumetric flask. 1. The mouth of the volumetric flask is very narrow. It is very likely that the student will spill the solid during transfer. 2. It is very difficult to agita te the mixture sufficiently to dissolve a solid in the volumetric flask. This is why only solutions, not solids, are added to the volumetric flask. After weighing the solid in the weighing bottle, dissolve the solid in a beaker before transfe rring the sol ution into the volumetric flask. Many students tried to dissolve the solid in the weighing bottle. The weighing bottle is very small with very little space to stir with a glass rod to dissolve the solid. Very often, some of the solution is spilt, leading to loss of the chemical. Transfer the solid from the weighing bottle to a beaker before trying to dissolve it. Some students used the method of difference to weigh the solid i.e. 1. Measure mass of empty weighing bottle with solid 2. Measure mass of emptied weighing bottle. 3. Take the difference of the two values to obtain the actual mass of solid used. This method is problematic when students rinse the emptied w eighing bottle with water before measuring the mass of the emptied weighing bottle. Rinsing the emptied weighing bottle effectively transfers all the solid into the beaker, making the use of the reweighing method unnecessary. More importantly, if the emptied weighing bottle is rinsed before its mass is measured, the mass would include water left over from the rinsing, making the mass calculated extremely unreliable. If you wish to use the method of difference, you should not r inse the weighing bottle with water. In this experiment, it is easier to use the tare method. B1(c) 1. During titration, as SCN– is added, Ag+ is removed. The position of equilibrium may shift left so as to counteract the change. Ag reacts with Fe 3+, producing more Ag+. This would lead to inaccurate titration results. This document is copyrighted, please do not reproduce it without permission

3 2. The actual volume of solution pipetted would be less than 25.0 cm 3 since the Ag(s) occupies some of the volume. This would lead to inaccurate titration results. Examiner’s Comments  Very few candidates scored both marks in this part due to various misconceptions. The most prevalent misconception involves understanding how a solid affects (or does not affect) the position of a system in equilibrium. Students should consult their tutors clarify this concept if they are unsure.  Other common problems / misconceptions are described below.  Many students failed to understand that with or without the Ag(s) accidentally pipetted, the mixture being pipetted is already at equilibrium and its concentrations are the equilibrium concentrations.  The Ag(s) is present as part of the original equilibrium mixture. Even if some of it is pipetted, the mixture is still at equilibrium. Many students incorrectly thought that the additional Ag(s) disturbed the system at equilibrium and went on to describe how the position of equilibrium shifted in response.  Students have to show understanding that the equilibrium is disru pted as the titration takes place as the Ag+ is being removed by reaction with SCN –, so the backwards reaction can take place , now that Ag(s) is present in the sample that was pipetted , producing more Ag+.  A handful of students suggested that Ag(s) may int erfere with the end point colour change observation. The solution before end point is originally cloudy (due to the solid AgSCN produced), so that small amount of Ag(s) accidentally transferred is unlikely to cause any significant colour differences.  Students are advised to communicate their ideas clearly. A reduction in the volume of solution pipetted does not change the concentration of the ions present in the solution. However it does mean that fewer moles of Ag + are being pipetted and titrated . So it leads to a lower calculated [Ag+] based on the titration data instead. B1(d) Yellow-green  Orange (yellow + red) or Blood red Examiner’s Comments  When asked for the colou