Chemistry 9701/31 October/November 2019: question paper and mark scheme

Question

In this experiment you will determine the concentration of a sample of hydrochloric acid. You will do this by measuring the volume of hydrogen produced when an excess of magnesium reacts with the acid. Mg(s) + 2HCl (aq) MgCl 2(aq) + H2(g) FA 1 is magnesium powder, Mg. FA 2 is hydrochloric acid, HCl. (a) Method ● Weigh the container with FA 1. Record the mass. ● Fill the tub with water to a depth of approximately 5 cm. ● Fill the 250 cm3 measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. ● Remove the paper towel and clamp the inverted measuring cylinder so that the open end is just above the base of the tub. ● Use the 25 cm3 measuring cylinder to place 25.0 cm3 of FA 2 into the reaction flask, labelled X. ● Check that the bung fits tightly in the neck of flask X, clamp flask X, and place the end of the delivery tube into the inverted 250 cm3 measuring cylinder. ● Remove the bung from the neck of flask X. Tip all of FA 1 into flask X and replace the bung immediately. Remove the flask from the clamp and swirl to mix the contents. ● Swirl the flask occasionally until no more gas is evolved. Replace the flask in the clamp. ● Measure and record the final volume of gas in the measuring cylinder. ● Weigh and record the mass of the container with any residual solid. ● Calculate and record the mass of FA 1 used. Keep FA 2 for use in Question 2. [2] (b) Calculations (i) Calculate the number of moles of hydrogen gas produced. (Assume 1 mol of gas occupies 24.0 dm3 at this temperature.) moles of H2(g) = …… mol [1] (ii) Calculate the concentration of hydrochloric acid in FA 2. concentration of HCl in FA 2 = …… mol dm–3 [1] (iii) In this experiment the magnesium powder was in excess. Calculate the mass of magnesium powder needed for complete reaction with all the hydrochloric acid in 25.0 cm3 of FA 2. mass of Mg = …… g [1] (c) A student suggested two modifications to the method in (a) to give a more accurate value for the concentration. For each suggestion, state whether you agree with the student and explain your answer. Suggestion 1: Use magnesium ribbon rather than powdered magnesium; keep the rest of the experiment the same. . …… . …… Suggestion 2: Use twice the mass of magnesium powder; keep the rest of the experiment the same. . …… . …… [2] (d) Another student carried out the experiment in (a) but used less magnesium than that calculated in (b)(iii). State and explain the effect this would have on the calculated concentration of hydrochloric acid in FA 2. . …… . …… . …… [1] [Total: 8] 2 In this experiment you will determine the concentration of FA 2 by titration using aqueous sodium hydroxide. HCl (aq) + NaOH(aq) NaCl (aq) + H2O(l) FA 2 is hydrochloric acid, HCl. FA 3 is 0.100 mol dm–3 sodium hydroxide, NaOH. methyl orange indicator (a) Method Dilution of FA 2 ● Fill the burette with FA 2. ● Run between 40.00 and 45.00 cm3 from the burette into the 250 cm3 volumetric flask. ● Record the volume used. ● Make the solution up to the 250 cm3 mark by adding distilled water. ● Shake the flask thoroughly to ensure mixing. ● Label this solution of hydrochloric acid FA 4. volume of FA 2 used = …… cm3 Titration ● Rinse the burette with distilled water and then with a little FA 4. ● Fill the burette with FA 4. ● Pipette 25.0 cm3 of FA 3 into a conical flask. ● Add several drops of methyl orange indicator. ● Perform a rough titration and record your burette readings. The rough titre is …… cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Make sure any recorded results show the precision of your practical work. ● Record in a suitable form all of your burette readings and the volume of FA 4 added in each accurate titration. I II III IV V VI VII VIII [8] (b) From your accurate titration results, obtain a value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 3 required …… cm3 of FA 4. [1] (c) Calculations (i) Give your answers to (ii), (iii) and (iv) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of hydrochloric acid that reacted with 25.0 cm3 of FA 3. moles of HCl = …… mol [1] (iii) Calculate the concentration of hydrochloric acid in FA 4. concentration of HCl in FA 4 = …… mol dm–3 [1] (iv) Calculate the concentration of hydrochloric acid in FA 2. concentration of HCl in FA 2 = …… mol dm–3 [1] (d) Calculate the maximum percentage error in the volume of FA 2 you added to the volumetric flask. maximum percentage error = …… % [1] (e) In Question 1 and Question 2 you have determined the concentration of FA 2 by two different methods. Each method used has possible sources of error, for example in Question 1 the largest source of error is escape of gas. Apart from this error, state and explain a source of error for each method. Question 1 …… . …… Question 2 …… . …… [2] [Total: 16] Qualitative Analysis Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. At each stage of any test you are to record details of the following: ● colour changes seen; ● the formation of any precipitate and its solubility in an excess of the reagent added; ● the formation of any gas and its identification by a suitable test. You should indicate clearly at what stage in a test a change occurs. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests for ions present should be attempted. 3 (a) FA 5 is a salt that contains two different cations and a single anion from those listed in the Qualitative Analysis Notes. (i) Place a small spatula measure of FA 5 in a hard-glass test-tube and heat gently. Do not inhale the fumes. Record all your observations. . …… . …… . …… . …… [2] (ii) Pour a 4 cm depth of distilled water into a boiling tube. Add the remaining FA 5 and stir carefully until the solid has dissolved. This solution is FA 6. Carry out the following tests on FA 6 and record your observations. test observations To a 1 cm depth in a test‑tube, add aqueous ammonia. To a 1 cm depth in a boiling tube, add aqueous sodium hydroxide, then warm the mixture. test observations To a 1 cm depth in a test‑tube, add aqueous barium nitrate or aqueous barium chloride, then add dilute hydrochloric acid or dilute nitric acid. [4] (iii) Identify the three ions in FA 5. FA 5 contains …… , …… and …… . [2] (b) A student carried out Qualitative Analysis tests on a hydrated salt, FA 7, and concluded that it contained the ions K+, Cr3+ and SO42–. The relative formula mass of FA 7 is 499.3. Determine the formula of FA 7. The formula of FA 7 is …… . [2] Question 3 continues on page 10. (c) FA 8 is a solution containing a single cation and a single anion, both of which are listed in the Qualitative Analysis Notes. (i) Carry out the following tests and record your observations. test observations To a 1 cm depth in a test‑tube, add a few drops of aqueous acidified potassium manganate(VII), then add starch indicator. To a 1 cm depth in a test‑tube, add aqueous sodium hydroxide. [2] (ii) Identify the two ions in FA 8. FA 8 contains …… and . …… . [1] (iii) Suggest an additional test you could carry out to confirm the presence of the anion in FA 8. Carry out this test and record your result. [2] (iv) Give the ionic equation for the reaction you carried out using FA 8 and sodium hydroxide. Include state symbols. . …… [1] [Total: 16] Qualitative Analysis Notes 1 Reactions of aqueous cations reaction with ion NaOH(aq) NH3(aq) aluminium, white ppt. white ppt. Al 3+(aq) soluble in excess insoluble in excess ammonium, no ppt. – NH4+(aq) ammonia produced on heating barium, faint white ppt. is nearly always no ppt. Ba2+(aq) observed unless reagents are pure calcium, white ppt. with high [Ca2+(aq)] no ppt. Ca2+(aq) chromium(III), grey-green ppt. grey-green ppt. Cr3+(aq) soluble in excess insoluble in excess copper(II), pale blue ppt. blue ppt. soluble in excess Cu2+(aq) insoluble in excess giving dark blue solution green ppt. turning brown on contact green ppt. turning brown on contact iron(II), with air with air Fe2+(aq) insoluble in excess insoluble in excess iron(III), red-brown ppt. red-brown ppt. Fe3+(aq) insoluble in excess insoluble in excess magnesium, white ppt. white ppt. Mg2+(aq) insoluble in excess insoluble in excess off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown manganese(II), on contact with air on contact with air Mn2+(aq) insoluble in excess insoluble in excess zinc, white ppt. white ppt. Zn2+(aq) soluble in excess soluble in excess

Question

Reactions of anions ion reaction carbonate, CO2 liberated by dilute acids CO3 2– chloride, gives white ppt. with Ag+(aq) (soluble in NH3(aq)) Cl –(aq) bromide, gives cream ppt. with Ag+(aq) (partially soluble in NH3(aq)) Br (aq) – iodide, gives yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) I –(aq) nitrate, NH3 liberated on heating with OH–(aq) and Al foil NO3–(aq) nitrite, NH3 liberated on heating with OH–(aq) and Al foil NO2–(aq) sulfate, gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids) SO42–(aq) sulfite, gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids) SO3 (aq)2–

Question

Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 gives a white ppt. with limewater (ppt. dissolves with excess CO2) chlorine, Cl 2 bleaches damp litmus paper hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint The Periodic Table of Elements Group 1 2 13 14 15 16 17 18 1 2 H He hydrogen helium Key 1.0 4.0 3 4 atomic number 5 6 7 8 9 10 Li Be atomic symbol B C N O F Ne lithium beryllium name boron carbon nitrogen oxygen fluorine neon 6.9 9.0 relative atomic mass 10.8 12.0 14.0 16.0 19.0 20.2 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar sodium magnesium aluminium silicon phosphorus sulfur chlorine argon 23.0 24.3 3 4 5 6 7 8 9 10 11 12 27.0 28.1 31.0 32.1 35.5 39.9 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 at www.cambridgeinternational.org after the live examination series. K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton 39.1 40.1 45.0 47.9 50.9 52.0 54.9 55.8 58.9 58.7 63.5 65.4 69.7 72.6 74.9 79.0 79.9 83.8 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 16 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon 85.5 87.6 88.9 91.2 92.9 95.9 – 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 56 57–71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 lanthanoids Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn caesium barium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon 132.9 137.3 178.5 180.9 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 – – – 87 88 89–103 104 105 106 107 108 109 110 111 112 114 116 actinoids Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Fl Lv francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium flerovium livermorium – – – – – – – – – – – – – 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 lanthanoids La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium 138.9 140.1 140.9 144.4 – 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.1 175.0 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 actinoids Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium – 232.0 231.0 238.0 – – – – – – – – – – – Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge