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In this question you will determine the concentration of iron(II) ions in FA 2. To do this you will do a titration using potassium manganate(VII) solution. The iron(II) ions, Fe^{2+}, are oxidised by the manganate(VII) ions, MnO_4^{-}.
5Fe^{2+}(aq) + MnO_4^{-}(aq) + 8H^{+}(aq) \rightarrow 5Fe^{3+}(aq) + Mn^{2+}(aq) + 4H_2O(l)
When all the Fe^{2+} ions have been oxidised, the presence of unreacted MnO_4^{-} ions causes the solution to become a permanent pink colour.
FA 1 contains 0.0200 mol dm^{-3} manganate(VII) ions, MnO_4^{-}.
FA 2 is a solution containing iron(II) ions, Fe^{2+}.
FA 3 is 1.0 mol dm^{-3} sulfuric acid, H_2SO_4.
(a) Method
• Fill the burette with FA 1.
• Use the pipette to transfer 25.0 cm^3 of FA 2 into the conical flask.
• Use the 25 cm^3 measuring cylinder to add 10 cm^3 of FA 3 to the conical flask.
• Add FA 1 from the burette into the conical flask until the solution becomes a permanent pink colour.
• Perform a rough titration and record your burette readings in the space below.
The rough titre is ..................... cm^3.
• Do as many accurate titrations as you think necessary to obtain consistent results.
• Make certain any recorded results show the precision of your practical work.
• Record in a suitable form below all of your burette readings and the volume of FA 1 added in each accurate titration.
Keep FA 2 to use in Question 3.
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value.
25.0 cm^3 of FA 2 required ....................... cm^3 of FA 1.
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate the number of moles of manganate(VII) ions present in the volume of FA 1 calculated in (b).
moles of MnO_4^{-} = ............................. mol
(ii) Calculate the number of moles of iron(II) ions present in 25.0 cm^3 of FA 2.
moles of Fe^{2+} = ............................. mol
(iii) Calculate the concentration, in mol dm^{-3}, of iron(II) ions in FA 2.
concentration of Fe^{2+} in FA 2 = ............................. mol dm^{-3}
(iv) FA 2 was prepared by dissolving hydrated ammonium iron(II) sulfate, (NH_4)_2Fe(SO_4)_2.6H_2O in distilled water. Calculate the mass of salt that would have to be dissolved in 1.00 dm^3 of water to prepare FA 2.
(A_r: H, 1.0; N, 14.0; O, 16.0; S, 32.1; Fe, 55.8)
mass of (NH_4)_2Fe(SO_4)_2.6H_2O = ............................. g
563
599
534
In this experiment you will measure the heat given out by the reaction of excess zinc with copper(II) sulfate solution and use this to estimate the concentration of the copper(II) sulfate.
$\text{Zn(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{Cu(s)}$
FA 4 is zinc powder.
FA 5 is aqueous copper(II) sulfate, CuSO$_4$.
(a) Method
Read through the instructions carefully and prepare a table below for your results before starting any practical work.
• Support the plastic cup in the 250cm$^3$ beaker.
• Use the 50cm$^3$ measuring cylinder to transfer 40cm$^3$ of FA 5 into the plastic cup.
• Measure and record the initial temperature of the solution in the plastic cup.
• Start the stopwatch. Measure and record the temperature of the solution every 30 seconds up to and including the temperature at 2 minutes. Stir the solution frequently.
• At time $t = 2\frac{1}{2}$ minutes, add all the powdered zinc to the solution in the plastic cup and stir the mixture.
• Record the temperature every 30 seconds from $t = 3$ minutes up to and including $t = 9$ minutes. Stir the solution constantly.
(b)
(i) On the grid opposite, plot the temperature ($y$-axis) against the time ($x$-axis). The scale for the temperature axis must allow you to plot a point with a temperature 5°C greater than the maximum temperature you recorded.
(ii) Draw the following best-fit straight lines on the graph.
• a line through the points between time $t = 0$ minutes and time $t = 2$ minutes
• a line through the points between time $t = 5$ minutes and time $t = 9$ minutes
• a vertical line at time $t = 2\frac{1}{2}$ minutes
(iii) Extrapolate the first two straight lines so that they intersect the vertical line at time $t = 2\frac{1}{2}$ minutes.
Use these extrapolated lines to determine the theoretical temperature change at time $t = 2\frac{1}{2}$ minutes.
change in temperature = ............ °C
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Use your answer to (b)(iii) to calculate the heat energy produced in the reaction.
(Assume that 4.2 J are required to increase the temperature of 1 cm$^3$ of solution by 1°C.)
heat energy produced = ............................ J
(ii) The molar enthalpy change, $\Delta H$, for the reaction shown below is $-219$kJmol$^{-1}$.
$\text{Zn(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{Cu(s)}$
Use this value and your answer to (i) to calculate the number of moles of copper(II) sulfate in your reaction.
moles of CuSO$_4$ = ............................ mol
(iii) Use your answer to (ii), to calculate the concentration of copper(II) sulfate, in mol dm$^{-3}$, in FA 5.
concentration of CuSO$_4$ = ............................ mol dm$^{-3}$
(d)
(i) Calculate the maximum percentage error in the highest temperature that you recorded in your results table.
maximum percentage error = ............................ %
(ii) A student suggested that the concentration of the copper(II) sulfate could be determined more accurately if a greater mass of zinc had been used.
Explain whether you agree with this student.
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(iii) A second student suggested that the concentration of the copper(II) sulfate could be determined more accurately if a smaller volume of copper(II) sulfate was used.
Explain whether you agree with this student.
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547
510
548
(a) In Question 1 you used FA 2. This solution was prepared from hydrated ammonium iron(II) sulfate, $(NH_4)_2Fe(SO_4)_2.6H_2O$. To a 1 cm depth of FA 2 in a test-tube, add a small spatula measure of sodium carbonate. Record your observations.
Solutions containing $Fe^{2+}$ ions can quickly be oxidised in air if they are prepared by dissolving the solid in distilled water. Use your observations to suggest what other substance was added to solid $(NH_4)_2Fe(SO_4)_2.6H_2O$ to prepare FA 2.
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(b) FA 6 is a mixture of two salts, each of which contains a single cation and a single anion from those listed in the Qualitative Analysis Notes on pages 10 and 11. Do the following tests and record your observations in the table below.
[Table_1]
(i) Place a small spatula measure of FA 6 in a hard-glass test-tube and heat strongly.
(ii) Place a small spatula measure of FA 6 in a test-tube and carefully add dilute sulfuric acid until the reaction is complete, then add aqueous sodium hydroxide.
(iii) To a 3 cm depth of distilled water in a boiling tube, add the remaining sample of FA 6. Stir and then filter the mixture into a clean boiling tube. You will use this solution for tests (iv)-(vi).
(iv) To a 1 cm depth of the solution from (iii) in a test-tube, add aqueous sodium hydroxide.
(v) To a 1 cm depth of the solution from (iii) in a test-tube, add aqueous ammonia.
(vi) To a 1 cm depth of the solution from (iii) in a test-tube, add aqueous barium chloride or aqueous barium nitrate.
(vii) Suggest possible identities for the ions present in FA 6.
cations ..................................................................................................
anions ......................................................................................................
(viii) Describe a further test that would allow you to determine exactly which anions are present. Explain your choice. Do not do this test.
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594
525
504
