AS & A Level Chemistry - 9701 Paper 3 2012 Summer Zone 4

(a) Method

Step 1
• Fill the burette labelled FB 1 with FB 1.
• Pipette 25.0 cm³ of FB 3 into a conical flask.
• Add about three drops of phenolphthalein.
• Perform 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 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 FB 1 added in each accurate titration.

Step 2
• Pipette 25.0 cm³ of FB 3 into a conical flask.
• Using a measuring cylinder, add about 25 cm³ of 2 mol dm⁻³ sulfuric acid, FB 4, to the flask.
• Place the conical flask on a tripod and gauze and heat to about 80 °C.
• Fill the burette labelled FB 2 with FB 2.
• Use an appropriate method to carefully transfer the hot conical flask onto a white tile under the burette.
• Titrate the mixture in the conical flask with FB 2 until a permanent pale pink colour is seen. If a permanent brown colour is seen, stop the titration and begin Step 2 again.
• Perform 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 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 FB 2 added in each accurate titration.

Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.

(b) (i) From your accurate titration results in Step 1, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

25.0 cm³ of FB 3 required …………… cm³ of FB 1.

(ii) Write an equation for the reaction between sodium hydroxide and ethanedioic acid to give sodium ethanedioate and water.

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(iii) Use your answer from (i) to calculate the number of moles of sodium hydroxide, FB 1, required to react with 25.0 cm³ of FB 3 in Step 1.

moles of NaOH = ......................... mol

(iv) Use your answer to (iii) to determine the number of moles of ethanedioic acid in 25.0 cm³ of FB 3.

moles of C₂O₄H₂ in 25.0 cm³ of FB 3 = ............. mol [2]

(c) (i) From your accurate titration results in Step 2, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

25.0 cm³ of FB 3 required …………… cm³ of FB 2.

(ii) Use your answer from (i) to calculate the number of moles of potassium manganate(VII), FB 2, required to react with 25.0 cm³ of FB 3 in Step 2.

moles of KMnO₄ = ......................... mol

(iii) The equation for the reaction between acidified manganate(VII) ions and ethanedioate ions is shown below.
$$2\text{MnO}_4^−(aq) + 5\text{C}_2\text{O}_4^{2−}(aq) + 16\text{H}^+(aq) \rightarrow 2\text{Mn}^{2+}(aq) + 10\text{CO}_2(g) + 8\text{H}_2\text{O}(l)$$
Calculate the total number of moles of ethanedioate ions in 25.0 cm³ of FB 3.

total moles of C₂O₄²⁻ in 25.0 cm³ of FB 3 = …………… mol

(iv) Use your answers to (b)(iv) and (c)(iii) to calculate the number of moles of ethanedioate ions which came from the sodium ethanedioate dissolved in 25.0 cm³ of FB 3.

moles of C₂O₄²⁻ from C₂O₄Na₂ in 25.0 cm³ of FB 3 = ……………. mol [4]

(d) (i) Use your answer to (b)(iv) to calculate the mass of ethanedioic acid, C₂O₄H₂, in 25.0 cm³ of FB 3.
[A_r: H, 1.0; C, 12.0; O, 16.0]
(If you were unable to answer (b)(iv), you may assume that the number of moles of ethanedioic acid is 6.51 × 10^–4 mol.)

mass of ethanedioic acid = ....................... g

(ii) Use your answer to (c)(iv) to calculate the mass of sodium ethanedioate, C₂O₄Na₂ in 25.0 cm³ of FB 3.
[A_r: C, 12.0; O, 16.0; Na, 23.0]
(If you were unable to answer (c)(iv), you may assume that the number of moles of sodium ethanedioate is 4.13 × 10^–4 mol.)

mass of sodium ethanedioate = ....................... g

(iii) Calculate the percentage by mass of sodium ethanedioate present in FB 3.

Percentage by mass of sodium ethanedioate present is ................... %. [4]

(e) (i) What is the maximum error in a single burette reading?

maximum error = .................. cm³

(ii) A student suggested that using a burette to measure the 25.0 cm³ of acid would give a more accurate result than using a pipette. The percentage error of a 25.0 cm³ pipette is 0.24 %. Is the student correct? Explain your answer.

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(f) A student decided to use a 25.0 cm³ pipette instead of a measuring cylinder to measure the volume of FB 4 in Step 2.
State and explain whether this alteration will improve the accuracy of the calculation of the percentage by mass of sodium ethanedioate in the mixture.

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Compounds FB 5, FB 6 and FB 7 contain the same non-metal but in three different oxidation states. You are provided with solid samples of FB 5, FB 6 and FB 7. Carry out the tests described below and record your observations in the table.

[Table_1]

(i) To 1 cm depth of dilute hydrochloric acid in a test-tube add a small spatula measure of FB 5.
(ii) To 1 cm depth of dilute sulfuric acid in a boiling tube add the same depth of aqueous potassium iodide. Add a small spatula measure of FB 5.
(iii) To 1 cm depth of dilute sulfuric acid in a test-tube add about ten drops of aqueous potassium manganate(VII). Add a small spatula measure of FB 5.
(iv) Place a small spatula measure of FB 6 into a hard glass test-tube. Heat the contents gently.
(v) Place a small spatula measure of FB 6 into a boiling tube. Dissolve the solid in 1 cm depth of distilled water. Add 1 cm depth of aqueous sodium hydroxide. Warm the mixture with care.
(vi) Place a small spatula measure of FB 7 into a hard glass test-tube. Heat the contents gently at first, then heat more strongly. Allow to stand for a few minutes
(vii) Place a small spatula measure of FB 7 into a boiling tube. Dissolve the solid in about 1 cm depth of distilled water. Add 1 cm depth of aqueous sodium hydroxide. Warm the mixture with care.

(i) From your observations in (a), identify the non-metal present in FB 5, FB 6 and FB 7.
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(ii) Suggest the oxidation state of the non-metal in FB 5 and FB 6.
The oxidation state of the non-metal in FB 5 is ..............................
The oxidation state of the non-metal in FB 6 is ..............................
(iii) Suggest the type of reaction occurring in (a)(iii).
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(c) Solid compounds containing $Fe^{2+}$ and $Ni^{2+}$ are usually green. One of these ions is present in FB 8 and the other in FB 9. Both FB8 and FB9 are aqueous solutions.
(i) Use the Qualitative Analysis Notes on page 10 to select two reagents that, used in separate tests, could identify the presence of the $Fe^{2+}$ ion.
The reagents are ....................................................... and .......................................................
(ii) Use your chosen reagents to carry out tests on both FB 8 and FB 9. Record your results in an appropriate form in the space below.
(iii) From the results of the tests in (ii), state which solution contains the iron(II) ions.
$Fe^{2+}$ ions are contained in solution .............................. .
Explain how your observations support your conclusion.
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(iv) Aqueous EDTA is a reagent used to identify some transition metals. To 1 cm depth of the solution containing the nickel(II) ion, add 1 cm depth of aqueous EDTA.
observation
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(v) State what you would expect to see if acidified potassium manganate(VII) was added to a sample of the solution containing the iron(II) ion. Do not carry out this experiment.
expected observation .......................................................
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