Chemistry 9701/31 May/June 2019: question paper and mark scheme

Question

Metal carbonates react with acid to produce carbon dioxide. You will determine the identity of a Group 2 metal M in a carbonate of formula MCO3 by reacting the carbonate with excess dilute hydrochloric acid and measuring the volume of carbon dioxide produced. MCO3(s) + 2HCl (aq) MCl 2(aq) + CO2(g) + H2O(l) FA 1 is 50 cm3 of 4.00 mol dm–3 hydrochloric acid, HCl. FA 2 is the metal carbonate, MCO3. (a) Method ●● 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 the open end is in the water just above the base of the tub. ●● Add all the FA 1 into the flask labelled X. ●● Check that the bung fits tightly into the neck of flask X, clamp flask X and place the end of the delivery tube into the inverted 250 cm3 measuring cylinder. ●● Weigh the container with FA 2 and record the mass. ●● Remove the bung from the neck of the flask. Tip the FA 2, from the container, into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. ●● Replace the flask in the clamp. Leave for several minutes, swirling the flask occasionally. You may wish to start Question 2 while the gas is being evolved. ●● When no more gas is collected, measure and record the final volume of gas in the measuring cylinder. ●● Weigh the container, with any residual FA 2, and record the mass. ●● Calculate and record the mass of FA 2 added to flask X. Results [3] (b) Calculations (i) Calculate the number of moles of carbon dioxide collected in the measuring cylinder. [Assume that 1 mol of gas occupies 24.0 dm3 under these conditions.] moles of CO2 = …… mol [1] (ii) Use your answer to (b)(i) and the information on page 2 to calculate the relative atomic mass, Ar, of M. Ar of M = …… [3] (iii) Use your answer to (b)(ii) to identify M. M is …… [1] (c) (i) A student suggested that, using the same apparatus, the accuracy of the experiment would be increased if approximately 2 g of MCO3 were used to react with the excess hydrochloric acid. State and explain whether the student was correct. . …… . …… . …… [1] (ii) Another student suggested that the experiment would be more accurate if the carbon dioxide was collected in a gas syringe rather than over water. State and explain whether the student was correct. . …… . …… . …… [1] [Total: 10]

Question

In Question 1 you measured the volume of carbon dioxide produced by a metal carbonate, MCO3, in order to identify M. In Question 2 you will identify another Group 2 metal, Q, by using a gravimetric method. When Group 2 carbonates are heated they decompose. QCO3(s) QO(s) + CO2(g) FA 3 is the metal carbonate, QCO3. (a) Method ●● Weigh the crucible with its lid and record the mass. ●● Add between 1.30 g and 1.50 g of FA 3 into the crucible. Record the mass of crucible, lid and FA 3. ●● Place the crucible on the pipe-clay triangle on the tripod. Put the lid on the crucible and heat gently for approximately 1 minute. ●● Use tongs to remove the lid and heat the crucible strongly for approximately 5 minutes. Replace the lid and then leave to cool. ●● While the crucible is cooling, begin work on Question 3. ●● When cool, reweigh the crucible with its lid and contents. Record the mass. ●● Calculate and record the mass of FA 3 placed in the crucible. ●● Calculate and record the mass of residue left after heating. Keep the crucible and its contents for use in Question 3(b). Results I II III IV [4] (b) Calculations (i) Calculate the number of moles of carbon dioxide produced during heating of FA 3. moles CO2 = …… mol [1] (ii) Use the mass of FA 3 in (a) and your answer to (b)(i) to calculate the relative atomic mass, Ar, of Q and hence identify Q. You should assume complete decomposition of QCO3. Ar of Q is …… Q is …… [4] (c) Explain why the lid was placed on the crucible when the residue was left to cool. . …… . …… [1] (d) In order to decompose Group 2 carbonates, the solid must be heated strongly. In this experiment QCO3 was heated for a few minutes. (i) Suggest an improvement to the method used that would ensure that decomposition was complete. . …… . …… [1] (ii) Suggest a chemical test to determine whether the decomposition of QCO3 was complete. State the expected observation if the decomposition was incomplete. Do not carry out this test. . …… . …… [1] (e) (i) In your calculation in (b) you used the mass of QCO3 and assumed that it was all decomposed during the heating. Explain what effect incomplete decomposition would have on the calculated value of the Ar of Q. . …… . …… . …… [1] (ii) A student suggested that you could use the mass of the residue, QO, rather than the mass of QCO3 in a calculation to identify Q. Explain why this method of calculating the Ar of Q is valid. . …… . …… [1] [Total: 14] 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.

Question

(a) FA 4 and FA 5 are aqueous solutions each containing one anion and one cation. (i) Carry out the following tests and record your observations. For each test use a 1 cm depth of FA 4 or FA 5 in a test-tube. observations test FA 4 FA 5 Add a 1 cm depth of dilute hydrochloric acid. Leave to stand. Add a 1 cm depth of aqueous copper(II) sulfate. Leave to stand. Add a few drops of aqueous silver nitrate, then add aqueous ammonia. Add a 1 cm depth of aqueous chlorine, then add a 1 cm depth of FA 5. [5] (ii) From your observations in (a)(i) identify one of the ions present in either FA 4 or FA 5. Ion present in …… is …… . [1] (iii) Apart from the reaction with FA 5 suggest a test that could be used to identify the coloured product formed in the reaction between aqueous chlorine and FA 4. You should include the reagent used and the expected observation. Do not carry out this test. reagent …… expected observation …… [1] (b) (i) Place the cooled crucible and residue from Question 2 onto a heatproof mat and add approximately 5 cm3 of water. Test the solution with litmus papers. Record your observations. . …… . …… [1] (ii) Using QO as the formula of the residue, write the equation for the reaction with water that occurs in (b)(i). Include state symbols. . …… [1] (c) In Questions 1 and 2 you identified the Group 2 metals present in MCO3 and QCO3. You will now plan and carry out tests to confirm, or not confirm, the identities of M and Q. Both M and Q are listed in the Qualitative Analysis Notes. (i) Group 2 carbonates are insoluble in water. In order to test for the cations present (M2+ and Q2+) they must be in solution. Name a reagent you could use to prepare solutions of the cations from solid samples of MCO3 and QCO3. . …… [1] (ii) You are provided with the following solutions. FA 6 contains M2+(aq). FA 7 contains Q2+(aq). Choose reagents that could be used to confirm the identity of M and Q. Carry out the tests. Record the tests, observations and conclusions. [5] (iii) Do your conclusions confirm your identification of M and Q in Questions 1 and 2? Explain your answer. . …… . …… . …… [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 2 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– 3 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 12 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