Chemistry 0620/62 October/November 2023: question paper and mark scheme

Question

Some seashells contain a mixture of the insoluble compounds calcium carbonate and silicon(IV) oxide only. Calcium carbonate reacts with dilute hydrochloric acid to form the soluble salt calcium chloride. Silicon(IV) oxide does not react with or dissolve in dilute hydrochloric acid. A student wants to find the percentage of silicon(IV) oxide in a seashell. The first four steps of the method the student uses are shown. step 1 The student grinds the seashell to form a powder. step 2 The student finds the mass of the powdered seashell. step 3 The student adds the powdered seashell to an excess of dilute hydrochloric acid and heats while stirring with a glass rod as shown in Fig. 1.1. step 4 The student filters the mixture as shown in Fig. 1.2. residue A B Fig. 1.1 Fig. 1.2 (a) Name the apparatus used to grind the seashell to form a powder in step 1. . …… [1] (b) Explain why it is important that the dilute hydrochloric acid is in excess in step 3. . …… . …… [1] (c) Name the items of apparatus labelled A and B in Fig. 1.2. A …… B …… [2] (d) The residue obtained in step 4 is not pure. (i) Identify one substance, other than water, that is in the residue and prevents it from being pure. . …… [1] (ii) The student washes the residue. Describe two additional steps the student must now take to find the percentage of silicon(IV) oxide in the seashell. . …… . …… . …… . …… [2] [Total: 7]

Question

A student investigates the temperature change when aqueous sodium hydroxide neutralises dilute hydrochloric acid. The equation for the reaction is shown. NaOH + HCl → NaCl + H2O The student does six experiments. Experiment 1 ● Fill a burette with 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. ● Fill a second burette with aqueous sodium hydroxide. ● Run some of the aqueous sodium hydroxide out of the burette so that the level of the aqueous sodium hydroxide is on the burette scale. ● Run 1.0 cm3 of dilute hydrochloric acid from the burette into a boiling tube. ● Run 9.0 cm3 of aqueous sodium hydroxide from the second burette into the same boiling tube. ● Stir the mixture with a thermometer and measure the highest temperature reached. ● Measure the pH of the mixture in the boiling tube. ● Rinse out the boiling tube with distilled water. Experiment 2 ● Run 2.0 cm3 of dilute hydrochloric acid from the burette into the boiling tube. ● Run 8.0 cm3 of aqueous sodium hydroxide from the second burette into the same boiling tube. ● Stir the mixture with a thermometer and measure the highest temperature reached. ● Measure the pH of the mixture in the boiling tube. ● Rinse out the boiling tube with distilled water. Experiment 3 ● Repeat Experiment 2 using 3.0 cm3 of dilute hydrochloric acid and 7.0 cm3 of aqueous sodium hydroxide. Experiment 4 ● Repeat Experiment 2 using 6.0 cm3 of dilute hydrochloric acid and 4.0 cm3 of aqueous sodium hydroxide. Experiment 5 ● Repeat Experiment 2 using 7.0 cm3 of dilute hydrochloric acid and 3.0 cm3 of aqueous sodium hydroxide. Experiment 6 ● Repeat Experiment 2 using 8.0 cm3 of dilute hydrochloric acid and 2.0 cm3 of aqueous sodium hydroxide. (a) Use the description of the experiments and the thermometer diagrams to complete Table 2.1. Table 2.1 thermometer volume of dilute volume of highest diagram when experiment hydrochloric aqueous sodium temperature pH highest temperature acid / cm3 hydroxide / cm3 reached / °C reached 30 1 1.0 25 11 20 30 2 2.0 25 11 20 40 3 3.0 35 11 30 40 4 6.0 35 1 30 30 5 7.0 25 1 20 30 6 8.0 25 1 20 [4] (b) Add a suitable scale to the y-axis in Fig. 2.1. Your scale should extend by 2 °C above your highest temperature in Table 2.1. Plot your results from Experiments 1 to 6 on the grid. Draw two straight lines through your points, one through the first three points and one through the last three points. Extend your straight lines so that they cross. highest temperature reached / C 1 2 3 4 5 6 7 8 volume of dilute hydrochloric acid / cm3 Fig. 2.1 [5] (c) The point on the graph where the two straight lines cross is where all of the aqueous sodium hydroxide reacts with all of the dilute hydrochloric acid to form a neutral solution. (i) Use your graph in Fig. 2.1 to deduce the volume of dilute hydrochloric acid and the volume of aqueous sodium hydroxide that react together to produce a neutral solution. Show your working on Fig. 2.1. volume of dilute hydrochloric acid = …… cm3 volume of aqueous sodium hydroxide = …… cm3 [3] (ii) Predict the pH of the solution in the boiling tube when the volumes in (c)(i) are mixed together. pH = …… [1] (iii) Deduce which solution, dilute hydrochloric acid or aqueous sodium hydroxide, is the most concentrated. Use your answer to (c)(i) to explain why. most concentrated solution . …… explanation …… . …… [1] (d) State how the pH and temperature recorded in each experiment would differ, if at all, if a polystyrene cup is used in place of the boiling tube. Explain any differences. pH …… temperature …… explanation …… . …… [3] (e) The volumes of the solutions used in these experiments were measured using a burette. Explain why a volumetric pipette could not be used instead of a burette in this experiment. . …… [1] [Total: 18]

Question

A student tests two substances: solid K and solid L. Tests on solid K The student dissolves solid K in water to form solution K. The student divides solution K into four portions. Table 3.1 shows the tests and the student’s observations for solution K. Table 3.1 tests observations test 1 To the first portion of solution K, add a few white precipitate drops of aqueous ammonia. test 2 To the second portion of solution K, add a pale purple solution few drops of acidified aqueous potassium manganate(VII). test 3 To the third portion of solution K, add 1 cm3 cream precipitate of dilute nitric acid followed by a few drops of aqueous silver nitrate. test 4 To the fourth portion of solution K, add aqueous the solution becomes orange chlorine. (a) (i) Identify two cations that test 1 shows could be in solid K. . …… . …… [2] (ii) Describe an additional test that could be carried out on solution K to confirm which of the two cations you have identified in (a)(i) is in solid K. Explain how the test will show which of these two cations is in solid K. test . …… explanation …… . …… [2] (b) Identify the anion in solid K. . …… [1] Tests on solid L Solid L is barium nitrate. Complete the expected observations. (c) The student carries out a flame test on solid L. observations …… [1] The student dissolves the remaining solid L in water to form solution L. The student divides solution L into three portions. (d) To the first portion of solution L, the student adds a piece of aluminium foil and 5 cm3 of aqueous sodium hydroxide and warms the mixture. The student tests for any gas produced. observations …… . …… [1] (e) To the second portion of solution L, the student adds 1 cm3 of dilute nitric acid and a few drops of aqueous silver nitrate. observations …… . …… [1] (f) To the third portion of solution L, the student adds 1 cm3 of dilute sulfuric acid. observations …… . …… [1] [Total: 9]

Question

The solubility of solid sodium sulfate in water changes as the temperature of the water changes. Plan an experiment to find out how the solubility of sodium sulfate in water changes with temperature. You are provided with sodium sulfate, distilled water and common laboratory apparatus. . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… . …… [6] Notes for use in qualitative analysis Tests for anions anion test test result carbonate, CO32– add dilute acid, then test for carbon effervescence, carbon dioxide dioxide gas produced chloride, Cl – acidify with dilute nitric acid, then white ppt. [in solution] add aqueous silver nitrate bromide, Br – acidify with dilute nitric acid, then cream ppt. [in solution] add aqueous silver nitrate iodide, I– acidify with dilute nitric acid, then yellow ppt. [in solution] add aqueous silver nitrate nitrate, NO3– add aqueous sodium hydroxide, ammonia produced [in solution] then aluminium foil; warm carefully sulfate, SO42– acidify with dilute nitric acid, then white ppt. [in solution] add aqueous barium nitrate sulfite, SO32– add a small volume of acidified the acidified aqueous potassium aqueous potassium manganate(VII) manganate(VII) changes colour 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 white ppt., insoluble in excess a 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 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, ppt. turns brown near surface on ppt. turns brown near surface on standing standing iron(III), Fe3+ red-brown ppt., insoluble in excess red-brown ppt., insoluble in excess zinc, Zn2+ white ppt., soluble in excess, giving white ppt., soluble in excess, giving a colourless solution a colourless solution Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 turns limewater milky chlorine, Cl 2 bleaches damp litmus paper hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint sulfur dioxide, SO2 turns acidified aqueous potassium manganate(VII) from purple to colourless Flame tests for metal ions metal ion flame colour lithium, Li+ red sodium, Na+ yellow potassium, K+ lilac calcium, Ca2+ orange-red barium, Ba2+ light green copper(II), Cu2+ blue-green