Cambridge A Level Chemistry 9701 — 2018 May/June Paper 3 · Variant 3

1 Acids are defined as substances that can donate hydrogen ions, H+, to bases. Monoprotic acids contain one H+ that can be donated per molecule. Diprotic acids contain two H+ that can be donated per molecule. You will determine by a titration method whether acid Z is monoprotic or diprotic. FA 1 is a solution containing 6.10 g dm–3 of acid Z. FA 2 is 0.105 mol dm–3 aqueous sodium hydroxide, NaOH. bromophenol blue indicator (a) Method ● Pipette 25.0 cm3 of FA 1 into a conical flask. ● Fill a burette with FA 2. ● Add several drops of bromophenol blue indicator to the conical flask. ● Carry out a rough titration and record your burette readings in the space below. 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 accuracy of your practical work. ● Record, in a suitable form below, all of your burette readings and the volume of FA 2 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FA 2 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 1 required .............................. cm3 of FA 2. [1] (c) Calculations (i) Calculate the number of moles of sodium hydroxide present in the volume of FA 2 calculated in (b). moles of NaOH = .............................. mol Then deduce the number of moles of H+ present in 25.0 cm3 of FA 1. moles of H+ in 25.0 cm3 of FA 1 = .............................. mol [1] (ii) Calculate the number of moles of H+ present in 1 dm3 of FA 1. moles of H+ in 1 dm3 of FA 1 = .............................. mol [1] (iii) FA 1 contains 6.10 g dm–3 of acid Z. The relative molecular mass of Z is 126. Calculate the number of moles of Z in 1 dm3 of FA 1. moles of Z in 1 dm3 of FA 1 = .............................. mol [1] (iv) Use your answers to (ii) and (iii) to determine whether Z is a monoprotic or a diprotic acid. Explain your answer. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 12]

Mark scheme: 1(a) 1 II All three headings and units correct for accurate titrations Headings: initial / final (burette) and reading / volume / vol or reading / volume / vol at start / finish (but not V) and volume / FA 2 and added / used or titre and Units: (cm3) or / cm3 or in cm3 [or cm3 by every entry] 1 III All accurate burette readings are recorded to the nearest 0.05 cm3 Do not award this mark if: • 50(.00) is used as an initial burette reading; • more than one final burette reading is 50(.00); • any burette reading is greater than 50(.00) • there is only one accurate titration 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. Do not award the mark if any ‘accurate’ burette readings (apart from initial 0) are given to zero dp. 1 For assessment of accuracy (Q marks) the examiner should round all burette readings to the nearest 0.05 cm3. Subtractions should be checked. The ‘best’ titres should be selected using the hierarchy: two (or more) identical; then 2 (or more) within 0.05 cm3; then two (or more) within 0.1 cm3 etc. These best titres should be used to calculate the mean corrected titre to the nearest 0.01 cm3 and this compared to the supervisor value. Award accuracy marks as shown below. V, VI and VII Award V, VI and VII for δ ⩽ 0.20 cm3 Award V and VI for 0.20 < δ ⩽ 0.30 cm3 Award V for 0.30 < δ ⩽ 0.50 cm3 3 Question Answer Marks 1(b) Candidate must average two (or more) accurate titres with total spread of not more than 0.20 cm3. Working must be shown or ticks must be put next to the two (or more) accurate readings selected. The mean should normally be quoted to 2 dp rounded to the nearest 0.01. Example: 26.667 must be rounded to 26.67. Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 eg 26.325; allow mean to 1 dp if all accurate burette readings were given to 1 dp and the mean is exactly correct. e.g. 26.0 and 26.2 = 26.1 is correct but 26.0 and 26.1 = 26.1 is incorrect. Do not award this mark if: the spread of selected titres is greater than 0.20 cm3; the rough titre was used to calculate the mean; the candidate carried out only 1 accurate titration; burette readings were incorrectly subtracted to obtain any of the accurate titre values; all burette readings, excluding initial 0, (resulting in titre values used in calculation of mean) are integers. Note: the candidate’s mean will sometimes be marked as correct even if it is different from the mean calculated by the examiner for the purpose of assessing accuracy. 1 1(c)(i) Correctly calculates: (b) × 0.105 / 1000 and moles H+ = moles NaOH and answers to 3 or 4 sf 1 1(c)(ii) Expression: H+ from (c)(i) × (1000 / 25) and answer to 3 or 4 sf 1 1(c)(iii) Correctly calculates 6.10 / 126 = 4.84(1) × 10‒2 and answer to 3 or 4 sf 1 1(c)(iv) Expression (c)(ii) / (c)(iii) and makes correct statement regarding FA 1 based on answer 1 Question Answer Marks

More questions on Acids and bases

2 When an acid reacts with an alkali the neutralisation reaction is always exothermic. H+(aq) + OH−(aq) H2O(l) You will determine the enthalpy change of neutralisation, ΔH, for a monoprotic acid X. FA 3 is aqueous sodium hydroxide, NaOH. FA 4 is a 2.00 mol dm–3 solution of monoprotic acid X. (a) Method ● Support the plastic cup in the 250 cm3 beaker. ● Fill the second burette with FA 3. ● Use the measuring cylinder to transfer 25.0 cm3 of FA 4 into the plastic cup. ● Measure and record the temperature of FA 4 in the plastic cup. ● Add 5.00 cm3 of FA 3 from the burette into the plastic cup. Stir the contents of the cup. Read and record the maximum temperature of the solution. ● Add a further 5.00 cm3 of FA 3 from the burette into the cup. Stir the contents of the cup. Read and record the maximum temperature of the solution. ● Repeat the addition of FA 3, in 5.00 cm3 portions, until 50.00 cm3 have been added. Read and record the maximum temperature of the solution after each addition. I II III IV [4] (b) On the grid plot a graph of temperature, (y-axis), against volume of FA 3 added, (x-axis). Your scale should allow a temperature of 2 °C above the maximum measured to be plotted. On your graph draw two lines of best fit. One line should be for when the temperature was rising and the other for after the maximum temperature had been reached. You should indicate clearly any points you consider to be anomalous. Extrapolate the lines so that they intersect. [4] (c) From your graph, determine the maximum temperature reached in the experiment and the volume of FA 3 added to produce this maximum temperature. maximum temperature reached = .............................. °C volume of FA 3 added to reach maximum temperature = .............................. cm3 [1] (d) (i) Calculate the energy released during this experiment. [Assume that 4.2 J of heat energy changes the temperature of 1.0 cm3 of solution by 1.0 °C.] energy released = .............................. J [1] (ii) Calculate the number of moles of acid X in 25.0 cm3 of FA 4. moles of X = .............................. mol [1] (iii) Calculate, in kJ mol–1, the enthalpy change of neutralisation for acid X. enthalpy change = ...... ............................. kJ mol–1 [1] (sign) (value) (e) Without changing the apparatus or solutions used, suggest one way in which the experiment could be modified to make the values obtained in (c) more accurate. .................................................................................................................................................... .............................................................................................................................................. [1] (f) The enthalpy change of neutralisation of hydrochloric acid with aqueous sodium hydroxide is more exothermic than the enthalpy change of neutralisation of acid X. Explain what this tells you about acid X. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [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 Table with appropriate headings to show 11 results Volume FA 3 or FA 3 / cm3 or burette readings and Temperature / Temp / T or ºC Allow initial temperature recorded on a separate line under method section. Allow volume added and allow vol / V for volume. The figures in this section may relate to total volume added (0, 5, 10, ...) or volume added each time (0, 5, 5, 5, ....) or be burette readings 1 II All temperatures recorded to .0 or .5 ºC All results follow the pattern (up and down) 1 Round thermometer readings to the nearest .5 ºC Examiner to calculate (and correct) supervisor’s ∆T – initial T to highest T recorded in table. Calculate candidate’s ∆T from table, calculate δ from supervisor and award marks as shown. III and IV Award III and IV if δ ± 1.0 ºC Award III if δ ± 2.5 ºC 2 2(b) Axes labelled and uniform scales chosen for plotted points (and T max plus 2 ºC) to occupy more than half the available space on both axes 1 All recorded points plotted (min of 8 recorded). Points must be accurate to within half a small square, be clearly on a line or not on a line (whichever is appropriate). If crosses used must be centred on correct point, not be large blobs. 1 Two lines of best fit drawn (can be straight or curved) – one for increase in temperature and the other for decrease in temperature. 1 Both lines are extrapolated to cross or to a discontinuity 1 2(c) Records correctly from graph candidate’s max T and vol FA 3 1 2(d)(i) Correctly uses: Energy = (vol from (c) + 25) × 4.2 × (max T – initial T) 1 2(d)(ii) Correctly calculates: moles X = 25 × 2.00 / 1000 = 0.05 1 Question Answer Marks 2(d)(iii) Expression: (d)(i) / ((d)(ii) × 1000) or correct answer and answer to 2 to 4 sf with negative sign 1 2(e) Repeat the experiment with volumes of FA 3 near to that needed for T max / near the end point 1 2(f) (X less exothermic than HCl shows) it is a weak acid/ shows X is not HCl / small(er) acid dissociation (constant) 1 Some of the energy that could be released is used to ionise HX 1

More questions on Enthalpy change, ΔH

3 (a) FA 5, FA 6 and FA 7 are 1.0 mol dm–3 sulfuric acid, 0.1 mol dm–3 sulfuric acid and 1.0 mol dm–3 hydrochloric acid but not necessarily in that order. You are to plan and carry out tests to determine the identities of FA 5, FA 6 and FA 7. You should record, in a suitable form in the space below, your tests and observations. You should show clearly the observations for each of FA 5, FA 6 and FA 7 with all test reagents. FA 5 is ......................................................................... . FA 6 is ........................................................................ . FA 7 is ........................................................................ . [4] (b) FA 8 and FA 9 each contain one anion and one cation. (i) Carry out the following tests and record your observations. For each test you should use a small spatula measure of FA 8 or FA 9. observations test FA 8 FA 9 Add a small spatula measure to a 1 cm depth of distilled water in a test‑tube and shake. Add a few drops of universal indicator. Record the pH of the mixture. Heat a small spatula measure in a hard-glass test‑tube. Dissolve a small spatula measure in a 2 cm depth of dilute hydrochloric acid in a boiling tube (heat gently if necessary). Place a 1 cm depth of this solution into two test-tubes. To one test‑tube, add aqueous sodium hydroxide. To the second test‑tube, add aqueous ammonia. [6] (ii) Mix a spatula measure of FA 8 with a spatula measure of FA 9. Heat the mixture in a hard-glass test-tube. Record your observations. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) From your observations in (b)(i) and (b)(ii) identify the ions present in FA 8 and FA 9. If you are unable to identify an ion, write ‘unknown’. FA 8 cation ..................................... anion ..................................... FA 9 cation ..................................... anion ..................................... [2] [Total: 13]

Mark scheme: 3(a) Choice of suitable reagents in table / clear layout. One reagent to show sulfate or chloride (BaCl2 or Ba(NO3)2 or AgNO3) One reagent to show different concentration of H+ in samples with sulfate (Mg, Zn, named carbonate) 1 White ppt with FA 7 and Ag+(aq) / white ppt with FA 5 and FA 6 with Ba2+(aq) and no (visible) reaction / no change / no ppt for others (ignore faint white ppt with Ag+) For each reagent used all three observations must be recorded and correct 1 Difference in rate of bubbling / fizzing / effervescence of FA 5 and FA 6 with suitable metal or carbonate Allow less and more fizz without time being mentioned 1 FA 5 is 0.1 mol dm‒3 H2SO4; FA 6 is 1.0 mol dm‒3 H2SO4; FA 7 is (1.0 mol dm‒3) HCl. 1 3(b)(i) 2* = 1 mark FA 8 FA 9 Blue / blue-green / blue-purple / dark green* pH 8–11* Yellow or orange-yellow or orange* pH 3–6* Water vapour /condensation / steam* Any two * from this box: sublimes / solid or white forms on cold part / top of tube / white smoke * Effect on litmus red to blue* Allow blue litmus to red on strong heating* White ppt / white solid* insoluble in excess* No (observable) reaction / no change / no ppt and White ppt / white solid* insoluble in excess * No (observable) reaction / no change / no ppt * 6 3(b)(ii) (gas / ammonia) turns (red) litmus blue 1 Question Answer Marks 3(b)(iii) any 2 correct answers for 1 mark all 4 correct answers for 2 marks FA 8 Mg2+ OH‒ FA 9 NH4 + unknown 2

More questions on General characteristic chemical properties of the first set of transition elements, titanium to copper