No questions found
You are to determine the concentration, in mol dm-3, of the aqueous sodium thiosulfate. To do this you will first produce iodine solution by reacting aqueous potassium iodide with aqueous potassium manganate(VII). In this reaction iodide ions are oxidised to iodine by manganate(VII) ions in acidic solution.
$2MnO_4^-(aq) + 10I^-(aq) + 16H^+(aq) \rightarrow 2Mn^{2+}(aq) + 5I_2(aq) + 8H_2O(l)$
You will then titrate the iodine with aqueous thiosulfate ions, in FB 1.
$2S_2O_3^{2-}(aq) + I_2(aq) \rightarrow S_4O_6^{2-}(aq) + 2I^-(aq)$
You are provided with the following.
- FB 1 is a solution of sodium thiosulfate, $Na_2S_2O_3$, of unknown concentration.
- FB 3 is 0.0050 mol dm-3 potassium manganate(VII), KMnO4.
- FB 4 is 0.10 mol dm-3 potassium iodide, KI.
- FB 5 is 1.0 mol dm-3 sulfuric acid, $H_2SO_4$.
(a) Method
**Dilution**
- Fill the burette with FB 1.
- Run between 45.50 cm3 and 46.50 cm3 of FB 1 from the burette into the 250 cm3 graduated (volumetric) flask, labelled FB 2.
- Make the solution up to the mark with distilled water.
- Shake the flask to mix the solution of FB 2.
**Titration**
- Fill a second burette with FB 2, the diluted sodium thiosulfate.
- Pipette 25.0 cm3 of FB 3 into a conical flask.
- Using a 25 cm3 measuring cylinder, add about 10 cm3 of FB 4.
- Using the same measuring cylinder, add about 10 cm3 of FB 5.
- Titrate the mixture in the flask with FB 2 until the colour is pale yellow.
- Add about 10 drops of starch indicator. A blue-black colour should be seen as the starch reacts with the remaining iodine.
- Continue to add FB 2 until the blue-black colour just disappears leaving a colourless solution.
The rough titre is ............... cm3.
- Carry out 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 an appropriate form below all of your burette readings and the volume of FB 2 added in each accurate titration.
(b) From your accurate titration results obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
The iodine produced by 25.0 cm3 of FB 3 required ............... cm3 of FB 2.
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate how many moles of $MnO_4^-$ were pipetted into the conical flask.
................. mol of $MnO_4^-$
(ii) Calculate how many moles of $I_2$ were produced from the number of moles of $MnO_4^-$ calculated in (i).
$2MnO_4^-(aq) + 10I^-(aq) + 16H^+(aq) \rightarrow 2Mn^{2+}(aq) + 5I_2(aq) + 8H_2O(l)$
................. mol of $I_2$
(iii) Calculate how many moles of $S_2O_3^{2-}$ reacted with the $I_2$ in (ii).
$2S_2O_3^{2-}(aq) + I_2(aq) \rightarrow S_4O_6^{2-}(aq) + 2I^-(aq)$
................. mol of $S_2O_3^{2-}$
(iv) Calculate how many moles of $S_2O_3^{2-}$ were present in the 250 cm3 graduated (volumetric) flask.
................. mol of $S_2O_3^{2-}$
(v) Use your answer to (iv) and the volume of FB 1 that was diluted to calculate the concentration, in mol dm-3, of the original solution of sodium thiosulfate, FB 1.
The concentration of $Na_2S_2O_3$ in FB 1 was ............... mol dm-3.
(d) The maximum error for a 25 cm3 pipette commonly used in schools is ±0.06 cm3. The maximum individual error in any single burette reading is ±0.05 cm3.
Calculate each of the following.
The maximum percentage error in the volume of FB 3 pipetted into the conical flask
maximum percentage error in the pipetted volume of FB 3 = .............. %
The maximum percentage error in the titre calculated in (b)
maximum percentage error in the titre volume = .............. %
508
599
593
You are to investigate how the rate of the following reaction varies with the concentration of sodium thiosulfate, Na₂S₂O₃.
Na₂S₂O₃(aq) + H₂SO₄(aq) → S(s) + Na₂SO₄(aq) + SO₂(g) + H₂O(l)
The rate can be found by measuring how long it takes for the solid sulfur formed to obscure the printing on the insert provided.
Care should be taken to avoid inhalation of SO₂(g) that is given off during this reaction.
FB 5 is 1.0 mol dm⁻³ sulfuric acid, H₂SO₄.
FB 6 is 0.10 mol dm⁻³ sodium thiosulfate Na₂S₂O₃.
Read through the instructions carefully and prepare a table for your results on page 7 before starting any practical work.
(a) Method
• Using the 50 cm³ measuring cylinder transfer 45 cm³ of FB 6 into a 100 cm³ beaker.
• Using the 25 cm³ measuring cylinder measure 10 cm³ of FB 5.
• Tip the FB 5 into the FB 6 in the beaker and immediately start timing.
• Stir the mixture once with a glass rod and place it on top of the printed insert.
• View the printed insert from above so that it is seen through the mixture.
• Record the time, to the nearest second, when the printing on the insert just disappears.
• Empty and rinse the beaker. Shake out as much of the water as possible and dry the outside of the beaker.
• You will repeat the experiment to find out how the time for the printing on the insert to disappear changes when a different volume of FB 6 is used.
• Using the 50 cm³ measuring cylinder transfer 20 cm³ of FB 6 and 25 cm³ of distilled water into the 100 cm³ beaker.
• Using the 25 cm³ measuring cylinder add 10 cm³ of FB 5 to the mixture and immediately start timing.
• Stir the mixture once with a glass rod and place it on top of the printed insert.
• View the printed insert from above so that it is seen through the mixture.
• Record the time, to the nearest second, when the printing on the insert just disappears.
• Select suitable volumes of FB 6 and distilled water for two further experiments to investigate the effect of volume of sodium thiosulfate on the time taken for the printing on the insert to just disappear.
Calculate the values of 1/time, where time is in seconds, to an appropriate number of significant figures.
In the space below, record, in an appropriate form, all measurements of volume, time, and your calculated values of 1/time.
[5]
(b) Why was the total volume of solution kept constant in the experiments?
...................................................................................................................
...................................................................................................................
...................................................................................................................
[1]
(c) It may be assumed that the rate of reaction is proportional to 1/time. Draw a conclusion from your results about the relationship between the concentration of sodium thiosulfate used and the rate of reaction. Explain your answer.
...................................................................................................................
...................................................................................................................
...................................................................................................................
[1]
(d) In the four experiments, which value of the time measured had the greatest error? Explain your answer.
...................................................................................................................
...................................................................................................................
...................................................................................................................
[1]
(e) How could the procedure be adapted to find the effect of changing the concentration of acid on the rate of reaction?
...................................................................................................................
...................................................................................................................
...................................................................................................................
[1]
570
588
523
There are two parts to this question. In the first part you will analyse three metal salts, FB 7, FB 8 and FB 9, to identify the cation in each. In the second part you will carry out a series of tests on a different salt, FB 10, to identify the anion.
(a) FB 7, FB 8 and FB 9 each contain one of aluminium ions, lead ions or zinc ions. By reference to the Qualitative Analysis Notes on pages 11 and 12, select two appropriate reagents to perform tests to identify the cations present.
reagent ..................................................................................................................................................
Record the tests performed and the results of those tests in an appropriate form in the space below.
[5]
(b) From your observations, identify the cations present in FB 7, FB 8 and FB 9. State the minimum evidence to support each of your choices.
FB 7 contains ............................ cations.
evidence ..................................................................................................................................................
..................................................................................................................................................................
FB 8 contains ............................ cations.
evidence ..................................................................................................................................................
..................................................................................................................................................................
FB 9 contains ............................ cations.
evidence ..................................................................................................................................................
..................................................................................................................................................................
[3]
(c) You are provided with a solid sample of FB 10 which is a metal salt. Use this sample to perform the experiments described below. Record all your observations in the table.
[Table_1]
(i) Place a spatula measure of solid FB 10 into a dry hard-glass tube. Hold the test-tube in a holder.
Heat the test-tube gently at first
and then very strongly for several minutes.
Allow the test-tube to cool, and then half fill it with distilled water. Dissolve the solid residue. This is FB 11. Retain this for tests (iv) and (v).
While you are waiting for the test-tube to cool, continue with experiment (c)(ii) and (iii).
(ii) Pour a 2 cm depth of aqueous copper(II) sulfate in a test-tube. Add a spatula measure of solid FB 10.
(iii) Pour a 2 cm depth of aqueous aluminium sulfate into a test-tube. Add a spatula measure of solid FB 10.
(iv) Pour a 2 cm depth of solution FB 11 into a test-tube.
Add the same volume of aqueous barium chloride.
Then, using a dropping pipette, add dilute hydrochloric acid until no further change is seen.
(v) Pour 1 cm depth of FB 11 into a test-tube and add an equal depth of aqueous copper(II) sulfate.
Transfer the contents of the test-tube into an evaporating dish and place it over a Bunsen burner on a tripod and gauze. Heat strongly until all the water has been driven off and no further change is seen.
(d) Consider your observations in (c) and answer the following questions. In each case, provide evidence from your observations to support your conclusions.
(i) Identify the anion present in FB 11 and state evidence to support your choice.
FB 11 contains the ............................ anion.
evidence ................................................................................................................................................................................................
(ii) Suggest what type of reaction occurs as FB 10 is converted into FB 11.
................................................................................................................................................................................................
(iii) Suggest an explanation for what you observed in (c)(iii).
................................................................................................................................................................................................
536
594
595
