Chemistry 0620/51 October/November 2020: question paper and mark scheme

Question

You are going to investigate the reaction between dilute ethanoic acid and two different solutions of sodium hydroxide labelled solution A and solution B. Read all of the instructions carefully before starting the experiments. Instructions You are going to do two experiments. (a) Experiment 1 ●● Rinse the burette with solution A. ●● Fill the burette with solution A. Run some of solution A out of the burette so that the level of solution A is on the burette scale. ●● Use the measuring cylinder to pour 25 cm3 of dilute ethanoic acid into the conical flask. ●● Add five drops of thymolphthalein indicator to the conical flask. ●● Record the initial burette reading in the table. ●● Slowly add solution A from the burette to the conical flask, while swirling the flask, until the solution just changes colour. ●● Record the final burette reading in the table and complete the table. Experiment 1 final burette reading / cm3 initial burette reading / cm3 volume of solution A added / cm3 Experiment 2 ●● Empty the conical flask and rinse it with distilled water. ●● Empty the burette and rinse it with distilled water. ●● Rinse the burette with solution B. ●● Fill the burette with solution B. Run some of solution B out of the burette so that the level of solution B is on the burette scale. ●● Use the measuring cylinder to pour 25 cm3 of dilute ethanoic acid into the conical flask. ●● Add five drops of thymolphthalein indicator to the conical flask. ●● Record the initial burette reading in the table. ●● Slowly add solution B from the burette to the conical flask, while swirling the flask, until the solution just changes colour. ●● Record the final burette reading in the table and complete the table. Experiment 2 final burette reading / cm3 initial burette reading / cm3 volume of solution B added / cm3 [4] (b) State the colour change observed in the conical flask in Experiment 2. from …… to …… [1] (c) Explain why universal indicator is not a suitable indicator to use in this titration. . …… . …… [1] (d) (i) State which solution of sodium hydroxide, solution A or solution B, is the more concentrated. Explain your answer. . …… . …… [1] (ii) State how many times more concentrated this solution of sodium hydroxide is compared to the other solution of sodium hydroxide. . …… . …… [1] (e) Determine the volume of solution B that would be required if Experiment 2 was repeated with 10 cm3 of dilute ethanoic acid. . …… . …… [2] (f) Describe how the reliability of the results could be checked. . …… . …… [1] (g) A 25 cm3 pipette can be used to measure the volume of a solution. (i) Describe an advantage of using a 25 cm3 pipette to measure the volume of the dilute ethanoic acid. . …… . …… [1] (ii) Explain why a 25 cm3 pipette could not be used to measure the volume of solution A. . …… . …… [1] (h) (i) Explain why the burette was rinsed with distilled water in Experiment 2. . …… . …… [1] (ii) Explain why the burette was then rinsed with solution B. . …… . …… [1] (iii) State the effect that not rinsing the burette with solution B would have on the final burette reading. Explain your answer. effect …… explanation …… . …… [2] [Total: 17]

Question

You are provided with two solids, solid C and solid D. Do the following tests on solid C and solid D, recording all of your observations at each stage. tests on solid C (a) Describe the appearance of solid C. . …… [1] (b) Place about half of solid C in a hard‑glass test‑tube. Heat the solid gently and then strongly. Record your observations. . …… . …… . …… . …… [3] (c) Place the remaining half of solid C in a boiling tube. Add about 10 cm3 of distilled water to the boiling tube. Stopper the boiling tube and shake it to dissolve solid C to form solution C. Divide solution C into two approximately equal portions in two test‑tubes. (i) Add a few drops of universal indicator solution to the first portion of solution C. Record your observations. . …… . …… [1] (ii) Add a spatula measure of solid sodium hydrogencarbonate to the second portion of solution C. Test any gas formed. Record your observations. . …… . …… . …… . …… [3] (d) What conclusions can you make about solid C? . …… . …… [2] tests on solid D Add solid D to about 10 cm3 of distilled water in a boiling tube. Stopper the boiling tube and shake it to dissolve solid D to form solution D. Divide solution D into four approximately equal portions in four test-tubes. (e) (i) Add a few drops of aqueous sodium hydroxide to the first portion of solution D. Record your observations. . …… [1] (ii) Now add an excess of aqueous sodium hydroxide to the mixture. Record your observations. . …… [1] (f) Add excess aqueous ammonia to the second portion of solution D. Record your observations. . …… [1] (g) Add about 1 cm depth of dilute nitric acid and a few drops of aqueous silver nitrate to the third portion of solution D. Record your observations. . …… [1] (h) Add about 1 cm depth of dilute nitric acid and a few drops of aqueous barium nitrate to the fourth portion of solution D. Record your observations. . …… [1] (i) Identify solid D. . …… [2] [Total: 17]

Question

A toothpaste contains: ●● sodium fluoride ●● calcium carbonate ●● silica ●● mint flavouring. Sodium fluoride and the mint flavouring are soluble in water. Calcium carbonate and silica are insoluble in water. Calcium carbonate reacts with dilute hydrochloric acid to form the soluble salt calcium chloride. Plan an investigation to find the percentage by mass of silica in the toothpaste. In your answer you should include how you will calculate the percentage by mass of silica in the toothpaste. You have access to normal 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