AS & A Level Chemistry - 9701 Paper 3 2014 Summer Zone 2

Carbon dioxide in the atmosphere dissolves in rain water making it acidic. When this water passes through limestone containing calcium carbonate, it forms solutions of calcium hydrogen carbonate.

In this experiment, you will determine the concentration of hydrogen carbonate ions in a solution by titration with hydrochloric acid.

$\text{FB 1}$ is $0.100 \, \text{mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$.
$\text{FB 2}$ contains an unknown concentration of hydrogen carbonate ions, $\text{HCO}_3^{-}$.
methyl orange indicator

(a) Method

• Fill the burette with $\text{FB 1}$.
• Pipette $25.0\,\text{cm}^3$ of $\text{FB 2}$ into a conical flask.
• Add a few drops of methyl orange indicator to the conical flask.
• Perform a rough titration and record your burette readings in the space below.

• 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 $\text{FB 1}$ added in each accurate titration.

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

As well as hydrogen carbonate ions, water that has passed through rocks also contains a range of metal ions such as Ca²⁺. The presence of Ca²⁺ ions leads to the water forming scum with certain types of soap. Soluble metal carbonates can be added to remove these ions. One such soluble carbonate is M₂CO₃·10H₂O.

In this experiment you will heat a sample of the carbonate salt to remove the water of crystallisation and leave the anhydrous salt, M₂CO₃. By working out how much water was lost you will be able to calculate the relative formula mass of M₂CO₃·10H₂O and hence identify M.

FB 3 is the hydrated salt, M₂CO₃·10H₂O.

(a) Method
• Weigh a clean, dry crucible and its lid.
• Transfer all the FB 3 into the crucible and put the crucible lid on.
• Reweigh the crucible, the lid and the contents.
• Place the crucible on the pipe-clay triangle.
• Heat the crucible very gently for 5 minutes.
• Remove the lid and heat strongly for a further 5 minutes.
• Allow the crucible to cool.
• You should start Question 3 while cooling is taking place.
• When the crucible is cool enough to handle, reweigh the crucible, the lid and the contents.
Record, in an appropriate form in the space below, all your weighings and calculations including the mass of FB 3 used and the mass of water lost.

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(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Use the mass of water lost on heating to calculate the number of moles of hydrated salt, M₂CO₃·10H₂O, in the initial sample.
[A_r: H, 1.0; O, 16.0]
moles of M₂CO₃·10H₂O = ..................... mol

(ii) Calculate the relative formula mass of M₂CO₃·10H₂O, FB 3.

relative formula mass of M₂CO₃·10H₂O = .............................

(iii) Identify M in the salt, FB 3.
[A_r: H, 1.0; Li, 6.9; C, 12.0; O, 16.0; Na, 23.0; K, 39.1; Rb, 85.5; Cs, 133]

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(c) (i) State the maximum error in the mass of FB 3 you recorded in (a).
maximum error in the mass of FB 3 = ................. g

(ii) Calculate the maximum percentage error in the mass of water lost in (a).
percentage error = .................%
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(d) Suggest a change to the method that would improve the accuracy of this experiment. Explain your answer.
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(a) Water in rivers and streams contains a range of different cations and anions. A particular salt occurs in some samples of spring water. You will carry out tests to identify this salt.

FB 4 is a solution of the salt. It contains one cation, which is listed in the Qualitative Analysis Notes on page 10. It also contains one anion, which is either sulfate, $SO_4^{2-}$, or sulfite, $SO_3^{2-}$.

(i) Choose reagents that will allow you to identify the cation in FB 4. Carry out suitable tests using these reagents and record your observations in the space below.

The cation in FB 4 is ...................... .

(ii) Choose reagents to identify the anion in FB 4. Carry out suitable tests using these reagents and record your results in the space below.

The anion in FB 4 is ...................... .

(b) FB 5, FB 6 and FB 7 are solutions each containing a single compound which could be ethanol, ethanal or propanone. To identify each compound you will react the samples with Tollens' reagent and with acidified potassium manganate(VII).

(i) Preparation of Tollens' reagent

● To approximately 2 cm depth of aqueous silver nitrate in a boiling tube, add approximately 0.5 cm depth of aqueous sodium hydroxide.

● Add aqueous ammonia a little at a time with continuous shaking until the brown precipitate just dissolves. Do not add an excess of ammonia.

| test | observations |
|------|--------------|
| To a 1 cm depth of each solution in a clean, dry test-tube add a few drops of the Tollens’ reagent that you have prepared. Do not shake the tube. If no reaction is seen, warm the tube in the hot water bath. | FB 5
FB 6
FB 7 |

When you have made your observations, rinse out all the test-tubes and the boiling tube which were used with Tollens’ reagent.

| To a 1 cm depth of each solution in a test-tube add a 1 cm depth of dilute sulfuric acid. Then add a few drops of aqueous potassium manganate(VII). If no reaction is seen, warm the tube in the hot water bath. | Do not carry out this test | |

(ii) Identify each compound.

FB ......... contains ethanol.
FB ......... contains ethanol.
FB ......... contains propanone.

(iii) Another reagent can be used with these samples to identify which compounds have a carbonyl group, $C=O$. This reagent has not been supplied so you cannot carry out this test. State the reagent and the expected observations of the three tests.

reagent: ...........................................................
expected observations

FB 5 .....................................................................................................................................................
FB 6 .....................................................................................................................................................
FB 7 .....................................................................................................................................................

(c) FB 8 is an aqueous solution of an organic compound. Carry out the following tests. You do not need to identify FB 8.

| test | observations |
|------|--------------|
| To a 1 cm depth of FB 8 in a test-tube add a 1 cm depth of dilute sulfuric acid. Then add a few drops of aqueous potassium manganate(VII). If no reaction is seen, place the test-tube in the hot water bath and leave to stand. | |

| To a 1 cm depth of FB 8 in a test-tube carefully add a small spatula measure of sodium hydrogen carbonate. | |