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.