2019 HCIS IB2 HL Chem Prelim P3 (ans)

2019 IB2 HL Chemistry Prelim Exam Paper 3 – Answers Section A 1. Rhubarb, spinach and beet are example of plants with high oxalate–content foods. Oxalic acid, H2C2O4 and its salts occur as end ‒products of metabolism in these plants. Consuming t hese plants may have some adverse effects in our bodies. The oxalate ion (C 2O42‒) binds with primarily calcium and can cause stone formation in the urinary tract. However to reduce the oxalate content, soaking these plants in hot water may possibly reduce the oxalate content by action of leaching. The solu tion containing these ions can then be quantitatively determined. Methods like redox titration with potassium permanganate, KMnO 4, can be used to determine the amount of oxalate ion in plants. The redox reaction between oxalate and permanganate ion is shown as below: 2MnO 4‒ + 16H + + 5C2O42‒ → 2Mn 2+ + 8H 2O + 10CO 2 (a) (i) Suggest a plausible research question for the a bove study mention. [1] How does increasing in temperature/ change in temperature of the water affect the leaching/extracting/ removal of oxalate ions from spinach/ rhubarb/ beet via redox titration with standard solution of KMnO 4./ How does duration of soaking affect the leaching/extracting/ removal of oxalate ions from spinach/ rhubarb/ beet via redox titration with standard solution of KMnO 4./ OWTTE Clear recognition, explicit mention of IV and DV is necessary for full marks. (ii) Identify two controlled variables in the abov e experiment. [2] Any two 1)Concentration of KMnO 4 2) Mass of Vegetable used 3) Type of spinach/ plants/ vegetable 3) Volume of water used to soak the water 4) Duration of soaking the vegetable in water / temperature of water (based on the IV above) (b) The standard solution of standard KMnO 4 was prepared fresh and kept in a dark bottle and the burette and flask were covered with alumini um foil. This was to minimise the decomposition of KMnO 4 by sunlight. Comment how this can affect the accuracy of experiment and explain how the volume of titre could be affected if one fails to do so . [2] If one fails to do the above, sunlight will decompose KMnO 4. Hence, there will be less KMnO 4 in the solution/ more KMnO 4 is required to react with oxalate ion. [1] The titre volume will be higher . [ ½] Hence, the oxalate ion calculated will be more than the actual amount in the spinach. [ ½]

− 2 − (c) After soaking the spinach in hot water for a period of time, the solution containing oxalate ion (extracted from 500.0 g of spinach) was transferred to a 250.0 cm 3 standard volumetric flask and the solution was made up to 250.0 cm 3 using tap water. White solid appeared and was observed to settle at the bottom of the flask. 25.0 cm 3 of standard oxalate solution was then pipetted and titrated against 0.001 mol dm ‒3 of KMnO 4. The average titre of 23.50 cm 3 of KMnO 4 was determined. (i) Calculate the amount of oxalate ion in 25.0c m 3 of oxalate ion. [2] Amount of KMnO 4 = 23.50/1000 X 0.001 =0.0000235 mol [1] Amount of Oxalate ion in 25.0 cm 3 = 5/2 X 23.50/1000 X 0.001 = 0.00005875 [1] (ii) State one assumption in your calculation and hence determine the concentration (ppm) of oxalate ion (C2O42‒) in 500.0 g of spinach. [2] Assumption: Leaching of oxalic from the spinach is equal to the amount of oxalic acid in plant. [1] The ppm of spinach = [Mass of oxalate ion( in mg)] / [mass of spinach ( in 1000g)] Amount of Oxalate in 250 cm 3 = 0.0005875 mole [1/2] Mr of Oxalate ion = 12.01X 2 + 16.01X 4 = 88.1 ( no unit) Mass of Oxalate = 0.05175g in 500.0g Mass of Oxalate in 1000g = 103 mg in 1000g Therefore concentration of oxalate ion in spinach = 103ppm [1/2] (iii) The solubilities of some oxalate salts are g iven below. Oxalate salt Concentration (g dm ‒3) MgC 2O4 12.5 Na 2C2O4 26.9 CaC 2O4 0.0067 Justify the identity of the white solid formed in (c). [2] CaC 2O4 [1] Based on the data, it is the lowest among all and hence easiest to be precipitated. [1] (iv) The formation of the white solid is a systematic limitation in this experiment. Suggest an improvement to overcome this. [1] Tap water contains ions ( like Ca, K and Na inside). Replaced this with de‒ionised water. [1] Reject filter the white solid. (d) In another experiment, oxalate ion was complexed wi th the Iron (II) ion to form a yellow solution. The concentration of oxalate ions in the resulting solution is then determined from

− 3 − a calibration curve, which is plotted by measuring the light absorbance of standard solutions. (i) Deduce the relationship between absorbance and concentration. [1] Absorbance is directly proportional to concentration of ethanoate ion (ii) By interpolation of the graph above, determine the concentration of oxalate ion, [C2O42-], when the absorbance measured is 0.08 A. [2] At absorbance of 0.80 A ,the concentration is 0.02 moldm -3 [1] Since it is the linear relationship, the concentration of C 2O42- = 0.02 x 1/10 = 0.002 moldm -3 [1] Section B: Option D – Medicinal chemistry 2. Carbamazepine,C15 H17 N2O, sold under the trade name Tegretol, is used primarily in the treatment of epilepsy and neuropathic pain. It is classified as a narrow therapeutic index drug . Common adverse effects include drowsiness, dizziness, headaches and migraines, motor coordination impairment, nausea, vomiting, and/or constipation. Synergistic effects with alcohol while taking carba mazepine may lead to enhanced depression of the central nervous system. Carbamazepine

− 4 − (a) Deduce the IHD of Carbamazepine. [1] 11 allow either approach (from MF or structure) (Accept 8.5 from above provided MF) (b) Comment on the solubility of the drug. [1] Contains bulky / no polar benzene/ phenyl rings, making the molecule less polar/ more non-polar Accept H-bond formation with water, [½] Sparingly soluble in water/ polar solvents OR soluble in non-polar solvents/ lipids. (Accepts any one correct answer) [½] DONOT accept highly soluble in water. (c) (i) Carbamazepine is tested by injection into mice to establish its LD 50 and ED 50 values. Distinguish between the two terms. [2] LD 50 is the amount/dose of a drug, which causes the death of 50% (one half) of a group of test animals/ OWTTE, must make reference to animal testing. [1] ED 50 is the amount/dose of a drug, which is required to achieve 50% of the desired response in 50% of the population/ OWTTE [1] (ii) State the significance of the term “Therapeut ic Index” in drug administration. [1] The larger value of TI indicates that there is a wide margin between the toxic and effective dose, whereas a smaller value indicates that there is a narrow margin between the effective and toxic dose./ OWTTE. OR Importance in determining the margin of safety of a drug. OR Important indicator for therapeutic drug monitoring. (d) Describe the term “synergistic effect”. [1] An interaction between two or more drugs taken at the same time that causes the total effect of the drugs to be greater than the sum of the individual effects of each drug/ owtte. (e) State one advantage of combinatorial chemistry in drug synthesis. [1] Any one • synthesis of large numbers of compounds using a variety of starting materials; • automated process reacts a small number of compounds with a variety of reagents; • mix-and-split technique to produce a large number of products; • library of many different but