Chemistry 0620/52 May/June 2022: question paper and mark scheme

Question

You are going to investigate the reaction between two different solutions of aqueous sodium carbonate, labelled solution K and solution L, and dilute hydrochloric acid using two different indicators. Read all of the instructions carefully before starting the experiments. Instructions You are going to do two experiments. (a) Experiment 1 ● Rinse a burette with water and then with the dilute hydrochloric acid for Question 1. ● Fill the burette with the dilute hydrochloric acid. Run some of the dilute hydrochloric acid out of the burette so that the level of the dilute hydrochloric acid is on the burette scale. Record the burette reading in the table. ● Use a measuring cylinder to pour 25 cm3 of solution K into a conical flask. ● Add five drops of methyl orange indicator and five drops of thymolphthalein indicator to the conical flask. ● Stand the conical flask on a white tile. ● Slowly add dilute hydrochloric acid from the burette to the conical flask, while swirling the flask, until the solution becomes yellow. This is the first colour change. Record the burette reading in the table. ● Continue to add dilute hydrochloric acid from the burette to the conical flask, while swirling the flask, until the solution changes colour again. This is the second colour change. ● Record the burette reading in the table and complete the table. Experiment 1 burette reading at first colour change / cm3 final burette reading at second colour change / cm3 initial burette reading / cm3 volume of dilute hydrochloric acid added for first colour change / cm3 total volume of dilute hydrochloric acid added for second colour change / cm3 [3] (b) Experiment 2 ● Empty the conical flask and rinse it with distilled water. ● Repeat Experiment 1 using solution L instead of solution K. Experiment 2 burette reading at first colour change / cm3 final burette reading at second colour change / cm3 initial burette reading / cm3 volume of dilute hydrochloric acid added for first colour change / cm3 total volume of dilute hydrochloric acid added for second colour change / cm3 [3] (c) State the second colour change observed in the conical flask in both experiments. (d) For Experiment 1, compare the volume of dilute hydrochloric acid needed for the first colour change with the volume of dilute hydrochloric acid for the second colour change. …… [2] (e) Compare the concentration of solution K used in Experiment 1 to the concentration of solution L used in Experiment 2. Explain your answer. …… [3] (f) (i) Deduce the volume of dilute hydrochloric acid needed for the second colour change when Experiment 2 is repeated using 50 cm3 of solution L. . …… [2] (ii) State why using 50 cm3 of solution L would cause a problem. . …… . …… [1] (g) State the advantage of using a pipette instead of the measuring cylinder in these experiments. …… [1] (h) Explain why the conical flask was swirled as the dilute hydrochloric acid was added from the burette. …… [1] (i) At the start of Experiment 1, the burette was rinsed with water and then with dilute hydrochloric acid. At the start of Experiment 2, the conical flask was rinsed with water but not with solution L. (i) Explain why the conical flask was rinsed with water. . …… . …… [1] (ii) Explain why the conical flask was not rinsed with solution L in Experiment 2. . …… . …… [1] [Total: 19]

Question

You are provided with solid M. Do the following tests on solid M, recording all of your observations at each stage. tests on solid M Add about 15 cm3 of distilled water to the boiling tube containing solid M. Stopper the boiling tube and shake the mixture to dissolve solid M and form solution M. Divide solution M into five approximately equal portions in five test‑tubes. (a) Test the pH of the first portion of solution M. pH = …… [1] (b) To the second portion of solution M add aqueous sodium hydroxide gradually until it is in excess and the test‑tube is at least half full. Keep the product for use in (c). Record your observations. …… [2] (c) (i) Transfer the product from (b) into a boiling tube. Add a piece of aluminium foil and warm the mixture gently. Test any gas produced. Record your observations. . …… . …… [2] (ii) Identify the gas made in (i). . …… [1] (d) To the third portion of solution M add about 1 cm depth of dilute nitric acid followed by a few drops of aqueous barium nitrate. Record your observations. …… [1] (e) (i) To the fourth portion of solution M add the zinc powder. Gently shake the test‑tube and then leave it to stand and settle for about 5 minutes. Keep the product for use in (ii). Record your observations. . …… . …… [1] (ii) Decant the solution formed in (i) into a test‑tube. Add aqueous ammonia gradually to the solution until there is no further change. Record your observations. . …… . …… [2] (f) (i) To the fifth portion of solution M add the aqueous potassium iodide. Keep the product for use in (ii). Record your observations. . …… [1] (ii) To the product from (i) add the aqueous sodium thiosulfate. Gently shake the test‑tube. As soon as you have made your observations pour the contents of the test‑tube into the ‘stop-bath’. Record your observations. . …… [1] (g) Use the results of the tests in (a) to (d) to identify three ions in solution M. …… [3] [Total: 15]

Question

The diagram shows some coffee beans. Caffeine occurs naturally in coffee beans. Caffeine is a white crystalline solid. It is very soluble in hot water but much less soluble in cold water. Plan an investigation to obtain a pure crystalline sample of caffeine from coffee beans. Assume that all other soluble substances in coffee beans are very soluble in both hot and cold water. You are provided with coffee beans and common laboratory apparatus. …… [6] Notes for use in qualitative analysis Tests for anions anion test test result carbonate (CO32–) add dilute acid effervescence, carbon dioxide produced chloride (Cl –) acidify with dilute nitric acid, then add white ppt. [in solution] aqueous silver nitrate bromide (Br –) acidify with dilute nitric acid, then add cream ppt. [in solution] aqueous silver nitrate iodide (I–) acidify with dilute nitric acid, then add yellow ppt. [in solution] aqueous silver nitrate nitrate (NO3–) add aqueous sodium hydroxide, then ammonia produced [in solution] aluminium foil; warm carefully sulfate (SO42–) acidify, then add aqueous white ppt. [in solution] barium nitrate sulfite (SO32–) add dilute hydrochloric acid, warm sulfur dioxide produced gently and test for the presence of will turn acidified aqueous sulfur dioxide potassium manganate(VII) from purple to colourless Tests for aqueous cations cation effect of aqueous sodium hydroxide effect of aqueous ammonia aluminium (Al 3+) white ppt., soluble in excess, giving a white ppt., insoluble in excess colourless solution ammonium (NH4+) ammonia produced on warming – calcium (Ca2+) white ppt., insoluble in excess no ppt., or very slight white ppt. chromium(III) (Cr3+) green ppt., soluble in excess grey-green ppt., insoluble in excess copper(II) (Cu2+) light blue ppt., insoluble in excess light blue ppt., soluble in excess, giving a dark blue solution iron(II) (Fe2+) green ppt., insoluble in excess green ppt., insoluble in excess iron(III) (Fe3+) red-brown ppt., insoluble in excess red-brown ppt., insoluble in excess zinc (Zn2+) white ppt., soluble in excess, giving a white ppt., soluble in excess, giving a colourless solution colourless solution Tests for gases Flame tests for metal ions gas test and test result metal ion flame colour ammonia (NH3) turns damp red litmus paper blue lithium (Li+) red carbon dioxide (CO2) turns limewater milky sodium (Na+) yellow chlorine (Cl 2) bleaches damp litmus paper potassium (K+) lilac hydrogen (H2) ‘pops’ with a lighted splint copper(II) (Cu2+) blue-green oxygen (O2) relights a glowing splint sulfur dioxide (SO2) turns acidified aqueous potassium manganate(VII) from purple to colourless