Cambridge A Level Chemistry 9701 — 2019 Feb/March Paper 3 · Variant 3

1 Several ores of copper contain both copper(II) carbonate and copper(II) hydroxide. This combination is called basic copper(II) carbonate. You will determine the composition of an ore of copper by reacting it with an excess of acid and collecting the gas evolved. CuCO3(s) + H2SO4(aq) CuSO4(aq) + H2O(l) + CO2(g) FA 1 is a sample of basic copper(II) carbonate. FA 2 is dilute sulfuric acid, H2SO4. The formula of basic copper(II) carbonate, FA 1, can be written as xCuCO3•yCu(OH)2. You will use your results to determine the ratio x : y in the formula. (a) Method ●● Fill the tub with water to a depth of about 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. ●● Use the 50 cm3 measuring cylinder to transfer 50 cm3 of FA 2 into the conical flask. ●● Fit the bung tightly in the neck of the flask, clamp the flask and place the end of the delivery tube into the inverted 250 cm3 measuring cylinder. ●● Weigh the container with FA 1 and record the mass. ●● Remove the bung from the neck of the flask. Tip FA 1 into the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. Swirl the flask occasionally until no more gas is produced. ●● Replace the flask in the clamp. ●● Reweigh the container with any residual solid and record the mass. ●● Calculate and record the mass of FA 1 added to the flask. ●● Measure and record the final volume of gas in the 250 cm3 measuring cylinder. Results [2] (ii) Calculate the number of moles of carbon dioxide collected in the measuring cylinder. [Assume 1 mole of gas occupies 24.0 dm3 under these conditions.] moles of CO2 = .............................. mol Hence deduce the number of moles of copper(II) carbonate in FA 1. moles of CuCO3 = .............................. mol [1] (iii) Calculate the mass of copper(II) carbonate in FA 1. mass of CuCO3 = .............................. g [1] (iv) Use your answer to (iii) and the mass of FA 1 added to the flask in (a) to calculate the mass of copper(II) hydroxide in FA 1. mass of Cu(OH)2 = .............................. g [1] (v) Hence calculate the mole ratio of the two components of basic copper(II) carbonate, FA 1. This is the ratio x : y. CuCO3 : Cu(OH)2 = 1 : ............. x : y [2] (c) How would the value of y calculated in (b) change if the experiment was carried out at a much lower temperature? Tick () the correct box. Explain your answer. y would decrease y would increase y would not change explanation ................................................................................................................................. .................................................................................................................................................... .................................................................................................................................................... [1] (d) Not all the carbon dioxide produced in the reaction is collected in the 250 cm3 measuring cylinder. One reason for this is that some carbon dioxide is lost before the bung can be replaced in the flask. Give one other reason why it is not possible to collect all of the carbon dioxide produced in (a). Suggest an improvement to the method to address this. reason ........................................................................................................................................ improvement .............................................................................................................................. .................................................................................................................................................... [1] [Total: 10]

Mark scheme: 1(a) I Clear layout for 4 data items with unambiguous headings and units covering all entries (2 weighings, 1 mass, 1 gas volume) 1 II Recording of volume of CO2 (collected) AND both weighings AND mass of FA 1 correctly calculated 1 1(b)(i) All answers to parts (ii) to (v) are given to 2−4 sf 1 1(b)(ii) Correctly calculates: V 24.0 1 000 × (a) AND (ii) = (i) 1 1(b)(iii) Correctly uses: (iii) = (ii) × 123.5 1 1(b)(iv) Correctly uses: candidate’s mass FA 1 – (iii) 1 1(b)(v) Correctly uses: moles Cu(OH)2 = (iv) / 97.5 1 Correctly uses: ratio (ii) : n(Cu(OH)2) = 1 : .................... Allow correctly rounded to nearest integer. 1 1(c) Ticks 2nd box (y would increase): lower T => smaller volume (of CO2) => smaller mass / moles / amount of CuCO3 OR lower T => more CO2 dissolves (=> less collected) => smaller mass / moles / amount CuCO3 Ticks 3rd box (y unchanged): lower T => molar gas volume will be smaller (compensates for smaller volume) 1 1(d) some CO2 dissolves (in water) so: hot water in tub / saturate water with CO2 initially / collect gas (directly) in gas syringe / use oil / non-polar solvent (in place of water) 1

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2 It is possible that ores containing basic copper(II) carbonate also contain water of crystallisation. The formula of these ores would be written as xCuCO3•yCu(OH)2•zH2O. In this experiment you will heat a sample of a different basic copper(II) carbonate which will thermally decompose as shown. xCuCO3•yCu(OH)2•zH2O(s) (x+y)CuO(s) + xCO2(g) + (y+z)H2O(g) You will use your results to determine whether this sample of a different basic copper(II) carbonate contains water of crystallisation. FA 3 is a sample of a different basic copper(II) carbonate. (a) Method ●● Weigh the empty crucible with its lid and record the mass. ●● Add all the FA 3 to the crucible. ●● Reweigh the crucible, lid and FA 3. Record the mass. ●● Support the crucible in the pipeclay triangle on top of the tripod. ●● Remove the lid. ●● Heat the crucible gently for about 1 minute and then strongly for about 4 minutes. ●● Replace the lid and allow the crucible to cool. ●● You may wish to start Question 3 while the crucible is cooling. ●● When the crucible has cooled, reweigh the crucible, lid and contents. Record the mass. ●● Calculate and record the mass of FA 3 used, the mass of residue and the loss of mass. Results I II III IV V VI [6] (b) Calculations (i) Assume the percentage by mass of copper(II) carbonate in FA 3 is 60.0%. Calculate the mass of copper(II) carbonate present in FA 3. mass of CuCO3 = .............................. g Hence calculate the number of moles of copper(II) carbonate in FA 3. moles of CuCO3 = .............................. mol [1] (ii) Use your results from (a) to calculate the number of moles of copper(II) oxide formed on heating FA 3. moles of CuO = .............................. mol [1] (iii) Use your answers to (i) and (ii) and the equation on page 5 to calculate the number of moles of copper(II) hydroxide in FA 3. moles of Cu(OH)2 = .............................. mol [1] (iv) Use your answer to (i) to calculate the mass of carbon dioxide produced by the thermal decomposition of the copper(II) carbonate in FA 3. mass of CO2 = .............................. g [1] (v) Use your answer to (iii) to calculate the mass of water produced by the thermal decomposition of the copper(II) hydroxide in FA 3. mass of H2O = .............................. g [1] (vi) Deduce whether water of crystallisation is present in basic copper(II) carbonate FA 3. Justify your answer using your results from (a) and your answers to (iv) and (v). ............................................................................................................................................. [1] (c) (i) The lid was replaced before the crucible was cooled. Explain how replacing the lid before the crucible was cooled may have increased the accuracy of your results. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Using the same apparatus, suggest an improvement to the method to increase the accuracy of your results. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) A student carried out the method in (a) and obtained inaccurate results. The student suggested that not all of the copper(II) carbonate in the sample of basic copper(II) carbonate FA 3 had thermally decomposed. Suggest a chemical test to determine whether the student was correct. Give the expected observations. Do not carry out this test. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 15] 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.

Mark scheme: 2(a) I Tabulates / clearly lays out all data 1 II Appropriate headings and units for all items listed in the table • mass (empty) crucible (+ lid) • mass crucible (+ lid) + FA 3 / contents before heating • mass crucible (+ lid) + contents after heating / residue • mass FA 3 • mass of residue • mass loss Units: / g, (g), in grams. Use of lid must be consistent. 1 III All recorded weighings to the same number of dp and all subtractions listed are correct (minimum 1) 1 Examiner checks subtractions then calculates and records the difference between the supervisor’s and candidate’s mass ratio: mass FA 3 ÷ mass loss (all to 2 dp) IV Award IV if δ ⩽ ±0.80 1 V Award V if δ ⩽ ±0.60 1 VI Award VI if δ ⩽ ±0.40 (Supervisor > 10.00, tolerances are 1.20, 0.90, 0.60; Supervisor > 20.00, tolerances are 2.00, 1.50, 1.00) 1 2(b)(i) Correctly calculates: mass CuCO3 from mass of FA 3 in (a) = 60.0 mass 3 100 × FA AND Correctly uses moles CuCO3 = answer/123.5 1 2(b)(ii) Correctly calculates: mass residue 79.5 from mass of residue in (a) 1 2(b)(iii) Correctly uses: (iii) = (ii) – (i) 1 Question Answer Marks 2(b)(iv) Correctly uses: mass CO2 = n(CuCO3) × 44 1 2(b)(v) Correctly uses: mass H2O = (iii) × 18 1 2(b)(vi) mass loss in (a) – [(iv) + (v)] / comparison of values AND appropriate comment on presence / absence of water 1 2(c)(i) prevents absorption of water (vapour) from atmosphere 1 2(c)(ii) heat to constant mass / heat for longer to ensure all CO2 and H2O driven off OR increase the mass (of FA 3) (to reduce % error) 1 2(c)(iii) add (named) acid AND fizz / effervescence / bubbling / gas gives white ppt with limewater (means student correct) OR no fizz indicates all CuCO3 decomposed OR reheat residue AND pass any gas through limewater which turns milky (means student correct) OR limewater does not turn milky indicates all CuCO3 decomposed 1

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3 FA 4 is a solid containing one cation and one anion. FA 5 is a solution containing one cation and one anion. Carry out the following tests and record your observations. (a) (i) Warm (do not boil) a 5 cm depth of FA 5 in a boiling tube. Stop warming the FA 5, add all of the FA 4 and shake the boiling tube. ............................................................................................................................................. Filter the mixture into a second boiling tube. The filtrate will be used in the tests in (ii). ............................................................................................................................................. [2] (ii) Use a 1 cm depth of the filtrate from (i) in separate test-tubes for each of the following tests. test observations Add aqueous ammonia. Add a 1 cm depth of aqueous potassium iodide, then add aqueous sodium thiosulfate. (Rinse the test-tube when you have completed this test.) Add a 1 cm depth of dilute nitric acid followed by a 1 cm depth of aqueous silver nitrate. Add a 1 cm depth of dilute hydrochloric or dilute nitric acid followed by a 1 cm depth of aqueous barium chloride or aqueous barium nitrate. [5] (iii) FA 6 is a dry sample of the residue obtained by filtration in (i). test observations Add a 1 cm depth of dilute nitric acid to all of the FA 6 in its test-tube. Allow the mixture to stand for about 1 minute, then add aqueous sodium hydroxide. [2] (b) (i) From your observations in (a), suggest the identity of the cation and the anion present in the filtrate produced in (a)(i). cation present in the filtrate ...................................................... anion present in the filtrate ....................................................... [1] (ii) Write an ionic equation for one reaction in (a)(ii) where a precipitate was formed. Include state symbols. ....................................................................................................................................... [1] (iii) State the type of reaction that occurred in the first part of (a)(iii). ....................................................................................................................................... [1] (c) A student suggested that FA 5 is an acid. Apart from using an indicator, suggest and carry out a chemical test to determine whether the student was correct. Record the name of the reagent you used, your observations and your conclusion. [3] [Total: 15] 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

Mark scheme: 3(a)(i) FA 4 + FA 5 observations may be in either order blue solution formed / colourless to blue / solution turns blue / blue filtrate 1 pink / brown / red-brown AND residue / solid 1 Question Answer Marks 3(a)(ii) test observations marks + NH3 ((pale) blue ppt) forming deep / dark blue solution in excess 1 + KI, then turns brown / yellow-brown 1 + Na2S2O3 white ppt 1 + HNO3, then + AgNO3 no (visible) reaction / no change / no ppt / remains a blue solution 1 + HCl / HNO3, then + BaCl2 / Ba(NO3)2 white ppt 1 5 3(a)(iii) FA 6 + nitric acid: (pale) blue solution 1 then + NaOH: (pale) blue ppt 1 3(b)(i) metal ion: Cu2+ / copper(II) AND anion: SO42– / sulfate 1 3(b)(ii) Ba2+(aq) + SO42–(aq) → BaSO4(s) OR Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s) OR 2Cu2+(aq) + 4I–(aq) → 2CuI(s) + I2(aq) / (s) 1 3(b)(iii) redox (from some reaction in (a)(iii)) OR oxidation of metal / Cu OR reduction of nitrate 1 3(c) Na2CO3 (or other named carbonate) / Mg / Al / Zn / Fe / sodium thiosulfate 1 + CO32−: effervescence / gas turns limewater milky / chalky / cloudy white / white ppt + appropriate metal: effervescence / gas pops with lighted splint + thio: white / off-white / pale yellow ppt 1 Student is correct / FA 5 is an acid from correct observation 1

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