Question 113 marks
Question
Many hydrated salts lose water of crystallisation when heated. You will identify the metal in a hydrated salt by heating the salt until it becomes anhydrous. The equation for the dehydration of the hydrated salt, X•16H2O, is shown. X•16H2O(s) → X(s) + 16H2O(g) FB 1 is the hydrated salt, X•16H2O. (a) Method ● Weigh a crucible with its lid and record the mass. ● Add between 1.8 g and 2.0 g of FB 1 to the crucible. ● Weigh the crucible and lid with FB 1 and record the mass. ● Place the crucible on the pipe-clay triangle. Gently heat the crucible and contents for approximately two minutes with the lid on. ● Remove the lid. Then heat the crucible and contents strongly for approximately four minutes. ● Replace the lid and leave the crucible and residue to cool for at least five minutes. While the crucible is cooling, you may wish to begin work on Question 2 or 3. ● Reweigh the crucible and contents with the lid on. Record the mass. ● Remove the lid. Heat the crucible and contents strongly for a further two minutes. ● Replace the lid and leave the crucible and residue to cool for at least five minutes. Then reweigh the crucible and contents with the lid on. Record the mass. ● Calculate and record the mass of FB 1 added to the crucible and the mass of residue obtained. I II III IV V [5] (b) Calculations (i) Calculate the number of moles of water of crystallisation lost during heating of FB 1. moles of H2O lost = …… mol [1] (ii) Use your answer to (b)(i) to calculate the number of moles of anhydrous residue, X, produced by the heating in (a). moles of X produced = …… mol [1] (iii) Calculate the relative formula mass of X. Mr of X = …… [1] (iv) X is the sulfate of a metal in Group 13 of the Periodic Table. Calculate the relative atomic mass of the metal. Show your working. Ar of the metal = …… [1] (v) Use your answer to (b)(iv) to identify the metal present in X. The metal is …… . [1] (c) (i) Suggest why the crucible and contents were heated with the crucible lid on for the first two minutes of the experiment. . …… . …… . …… [1] (ii) Suggest whether the experiment would be more accurate if you reheated the crucible and contents strongly for a third time. Explain your answer. . …… . …… . …… [1] [Total: 12] 2 In this experiment, you will determine the concentration of an alkali. You will mix different volumes of acid with a fixed volume of alkali and measure the temperature rise that occurs each time. You will then determine the enthalpy change for the neutralisation of the acid with the alkali. FB 2 is aqueous sodium hydroxide, NaOH. FB 3 is 1.95 mol dm–3 sulfuric acid, H2SO4. (a) Method ● Use the thermometer to measure and record the initial temperature of FB 2. initial temperature of FB 2 = …… °C ● Support a plastic cup in the 250 cm3 beaker. ● Fill one burette with FB 3. Label this burette FB 3. ● Fill the other burette with distilled water. Experiment 1 ● Use the 10 cm3 pipette to transfer 10.0 cm3 of FB 2 into the plastic cup. ● Add 8.00 cm3 of distilled water from the burette into the plastic cup. ● Add 2.00 cm3 of FB 3 from the burette into the plastic cup. ● Stir the mixture and measure the maximum temperature reached. (You may need to tilt the cup so that the bulb of the thermometer is completely immersed.) Record the maximum temperature. ● Empty, rinse and shake dry the plastic cup, ready for use in Experiment 2. ● Repeat this procedure to carry out experiments 2 to 5, using the volumes of FB 2, water and FB 3 shown in the table. Record the maximum temperature reached in each experiment. volume of volume of volume of maximum experiment FB 2 / cm3 H2O / cm3 FB 3 / cm3 temperature / °C 1 10.0 8.00 2.00 2 10.0 6.00 4.00 3 10.0 4.00 6.00 4 10.0 2.00 8.00 I 5 10.0 0.00 10.00 II 6 10.0 III IVCarry out one further experiment which will enable you to determine more precisely the V minimum volume of FB 3 that gives the highest maximum temperature. This is Experiment 6. Record the volumes of water and FB 3 and the maximum temperature in the table above.[5] (b) On the grid opposite, plot a graph of maximum temperature reached on the y-axis and volume of FB 3 on the x-axis. Select a scale on the y-axis which includes a temperature 3.0 °C above the maximum temperature reached. Label any points you consider to be anomalous. Draw two straight lines of best fit on your graph. The first line is for increasing maximum I temperature and the second after the maximum temperature was reached. II Extrapolate the two lines so that they intersect. III Use your graph to determine the volume of FB 3 that reacts with 10.0 cm3 of FB 2. IV volume of FB 3 = …… cm3 [4] (c) (i) Calculate the change in energy when the volume of FB 3 in (b) is neutralised by FB 2, sodium hydroxide. Assume that 4.2 J of energy changes the temperature of 1.0 cm3 of solution by 1.0 °C. energy change = …… J [1] (ii) Calculate the number of moles of sulfuric acid in the volume of FB 3 in (b). (If you were unable to answer 2(b), use 5.70 cm3 as the volume of FB 3.) moles of H2SO4 = …… mol [1] (iii) Calculate the enthalpy change of neutralisation, in kJ mol–1, for 1.00 mol of H2SO4 reacting with FB 2. enthalpy change of neutralisation = …… …… kJ mol–1 [1] sign value (iv) Write the equation for the neutralisation of FB 3 with FB 2. Include state symbols. . …… [1] (v) Use your answer to (c)(ii) and the information on page 4 to calculate the concentration, in mol dm–3, of NaOH in FB 2. concentration of NaOH in FB 2 = …… mol dm–3 [1] (d) Apart from using a more accurate thermometer, better insulation or taking more readings, suggest one modification to the procedure which would make the value for the enthalpy change of neutralisation calculated in (c)(iii) more accurate. . …… . …… . …… [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. 3 The following information about the redox properties of some anions will be helpful. anion property nitrite easily oxidised nitrate cannot be oxidised sulfite easily oxidised sulfate cannot be oxidised FB 4 and FB 5 are solutions each containing one cation and one anion. Both anions are listed in the Qualitative Analysis Notes. (a) Carry out the tests and record your observations in the table. Use a 1 cm depth of FB 4 or FB 5 in a test-tube for each test. observations test FB 4 FB 5 Test 1 Add an equal volume of aqueous sodium carbonate. Test 2 Add aqueous ammonia. Test 3 Add a few drops of aqueous barium nitrate (or aqueous barium chloride). Test 4 Add an equal volume of dilute nitric acid. Allow to stand for one minute, then add a few drops of aqueous silver nitrate. [4] (b) Carry out the following tests in boiling tubes and record your observations in the table. Use a 1 cm depth of FB 4 or FB 5 for each test. observations test FB 4 FB 5 Test 1 Add aqueous sodium hydroxide, then warm the mixture gently and carefully, then add one piece of aluminium foil to the mixture. [3] (c) Using the information given at the start of the question, select one further test to enable you to identify the anions present in each of FB 4 and FB 5. ● State the reagent(s) you will use for this test. ● Explain why this test will enable you to identify the anions in FB 4 and FB 5. ● Carry out your test and record the observations. reagent(s) …… explanation …… . …… observations [3] (d) Write the formulae of the anions and cations present in FB 4 and FB 5. If the tests you carried out did not allow you to identify the ion, write 'unknown'. FB 4: cation …… anion …… FB 5: cation …… anion …… [2] (e) Give the ionic equation for one precipitation reaction you observed when using FB 4. Include state symbols. . …… [1] [Total: 13] 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
1(a) I Four weighings written in the space provided 1 AND All weighings are recorded to the same number of decimal places (minimum one dp). Headings are not required for this mark but some indication needed regarding to what they refer. II Appropriate headings and units 1 (minimum four in the space provided and one must be for after heating) • mass of crucible and lid / g • mass of crucible, lid and FB 1 / g • mass of crucible, lid and residue after first heating / g • mass of crucible, lid and residue after second heating / g • mass of FB 1 (used) / g • mass of residue / anhydrous salt obtained / g Allow omission of lid but its use or omission must be consistent Allow ‘solid’ or ‘contents’ or ‘anhydrous FB 1’ for ‘residue’ Reject ‘FB 1’ for ‘residue’ Reject ‘weight’ Allow units displayed as / g or (g) or in gram(me)s in general mass heading or after every entry / heading III Masses of FB 1 and of residue are correctly subtracted 1 AND Mass after 2nd heating is within 0.05 g of mass after first heating Note: the mass of residue must be calculated from the lower balance reading after heating Note: if a one dp balance was used, the two masses must be identical Check all subtractions, and use the corrected values (if necessary) Calculate the ratio for supervisor mass FB1 / mass residue to 2 dp Write this value, ringed, on each candidate’s script. Calculate the candidate’s ratio mass FB1 / mass residue to 2 dp Calculate the δ value, the difference between these two ratios 1(a) Award IV only if 0.05 < δ ⩽ 0.10 1 Award V if δ ⩽ 0.05 1 Default value for Al 2(SO4)3•16H2O = 1.84 Default value for Al 2(SO4)3•18H2O = 1.95 1(b)(i) Correctly calculates no of moles of water 1 Moles of water = correct mass loss / 18 AND Answer is given to 2–4 significant figures Do not penalise sf more than once in 1(b). 1(b)(ii) Correct use of (b)(i) to calculate no of moles of residue 1 (b)(i) moles of residue = / 16 AND answer given to 2–4 sf 1(b)(iii) Correct use of b(ii) to calculate Mr of anhydrous salt 1 Mr = mass of residue / (b)(ii) AND answer given to 2–4 sf Allow ecf from calculated value in 1(a). 1(b)(iv) Correct expression to calculate Ar of metal 1 Ar = [(b)(iii) – 3 × 96.1] / 2 AND answer given to 3 or 4 sf 96.1 or 288.3 or 32.1 (or equivalent) must be seen in working. Allow expression to be shown in two (or more) steps Reject any extra working 1(b)(v) Identity of metal (aluminium). Metal must be an element in Group 13. 1 The metal must have the Ar closest to the value calculated. 0 < B* ⩽ 18.9 : 18.9 ⩽ Al ⩽ 48.3 : 48.4 ⩽ Ga ⩽ 92.2 : 92.3 ⩽ In ⩽ 159.6 : 159.6 ⩽ Tl ⩽ 300. * Allow B if the Ar is appropriate even though it is a non-metal. Allow Al 3+ for Al , etc. 1(c)(i) Lid prevents solid spitting or frothing out of crucible 1 Allow FB 1 / crystals / salt for solid 1(c)(ii) No: because the final two readings were [almost] identical and showing that no more water can be driven off. 1 OR Yes: because the final two readings were not equal and so it shows that more water still needs to be driven off. Allow No: because the residue mass changes and anhydrous salt / residue may decompose. Allow Yes: I need to reheat to check the accuracy as the mass has increased (on 2nd heating) Allow absence of No / Yes provided the answer can be inferred from the explanation. Reject ‘yes’ if final two weighings are identical.