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You are to determine the concentration of hydrochloric acid, which supplies the H⁺ ions in the following reaction.
$ \text{IO}_3^- \text{(aq)} + 5\text{I}^- \text{(aq)} + 6\text{H}^+ \text{(aq)} \rightarrow 3\text{H}_2\text{O} (l) + 3\text{I}_2 \text{(aq)} $
In the presence of an excess of $ \text{IO}_3^- $ ions and an excess of $ \text{I}^- $ ions, the amount of $ \text{I}_2 $ liberated is directly proportional to the amount of $ \text{H}^+ $ ions present and can be determined by titration with sodium thiosulfate, $ \text{Na}_2\text{S}_2\text{O}_3 $.
You are provided with the following reactants.
- FA 1 hydrochloric acid
- FA 2 containing 15.0 g dm⁻³ sodium thiosulfate, $ \text{Na}_2\text{S}_2\text{O}_3\cdot 5\text{H}_2\text{O} $
- aqueous potassium iodate(V), $ \text{KIO}_3 $
- aqueous potassium iodide, $ \text{KI} $
(a) Method
- Fill a burette with FA 2.
- Pipette 25.0 $ \text{cm}^3 $ of FA 1 into the conical flask.
- Use a 25 $ \text{cm}^3 $ measuring cylinder to add to the flask 10 $ \text{cm}^3 $ of aqueous potassium iodate(V) and 10 $ \text{cm}^3 $ of aqueous potassium iodide. There is an excess of each of these reagents.
- Place the flask on a white tile.
- Titrate the liberated iodine with FA 2.
- During the titration the colour of the iodine in the solution will fade from red-brown to orange to yellow. The end-point occurs when the solution just goes colourless with the addition of a single drop of FA 2.
- You should perform a rough titration. In the space below record your burette readings for this rough titration.
- Carry out as many accurate titrations as you think necessary to obtain consistent results.
- Record in a suitable form below all of your burette readings and the volume of FA 2 added in each accurate titration.
- Make certain any recorded results show the precision of your practical work.
The rough titre is .......................................... $ \text{cm}^3 $
(b) From your titration results obtain a suitable value to be used in your calculation. Show clearly how you have obtained this value.
25.0 $ \text{cm}^3 $ of FA 1 require .............. $ \text{cm}^3 $ of FA 2.
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate the concentration, in mol dm⁻³, of the sodium thiosulfate in FA 2. FA 2 contains 15.0 g dm⁻³ $ \text{Na}_2\text{S}_2\text{O}_3\cdot 5\text{H}_2\text{O} $.
[$ \text{A}_{\text{r}} $: H, 1.0; O, 16.0; Na, 23.0; S, 32.1]
The concentration of sodium thiosulfate in FA 2 is ............................ mol dm⁻³.
(ii) Calculate how many moles of $ \text{Na}_2\text{S}_2\text{O}_3 $ are contained in the volume of FA 2 recorded in (b).
............... mol of $ \text{Na}_2\text{S}_2\text{O}_3 $
(iii) Calculate how many moles of iodine, $ \text{I}_2 $ reacted with the $ \text{Na}_2\text{S}_2\text{O}_3 $ in (ii).
$ 2\text{Na}_2\text{S}_2\text{O}_3(\text{aq}) + \text{I}_2(\text{aq}) \rightarrow \text{Na}_2\text{S}_4\text{O}_6(\text{aq}) + 2\text{NaI}(\text{aq}) $
........................... mol of iodine reacted with the sodium thiosulfate.
(iv) Calculate how many moles of hydrochloric acid, HCl reacted with an excess of potassium iodate(V) and an excess of potassium iodide to produce the amount of iodine calculated in (iii).
$ \text{IO}_3^- (\text{aq}) + 5\text{I}^- (\text{aq}) + 6\text{H}^+ (\text{aq}) \rightarrow 3\text{H}_2\text{O}(l) + 3\text{I}_2(\text{aq}) $
........................... mol of HCl produced the amount of iodine calculated in (iii).
(v) Calculate the concentration, in mol dm⁻³, of HCl in FA 1.
The concentration of HCl in FA 1 is ...................... mol dm⁻³.
(d) Each reading with a burette has a maximum error of $ \pm 0.05 \text{cm}^3 $. Grade B volumetric (bulb) pipettes are calibrated to $ \pm 0.06 \text{cm}^3 $.
(i) Calculate the maximum error in the volume run from the burette recorded in any titration.
The maximum error is ............................. $ \text{cm}^3 $.
(ii) Express the maximum error calculated in (i) as a percentage error for the volume calculated in (b).
The maximum error is ............................... %.
(iii) Calculate the percentage error when 25.0 $ \text{cm}^3 $ of FA 1 was pipetted into the conical flask.
The error was ......................................... %.
569
532
549
FA 3 is powdered basic copper(II) carbonate, a hydrated mixture of copper(II) carbonate and copper(II) hydroxide.
The approximate formula for the basic carbonate is $\text{CuCO}_3\cdot\text{Cu(OH)}_2\cdot\text{H}_2\text{O}$.
When heated, basic copper(II) carbonate decomposes.
$$\text{CuCO}_3\cdot\text{Cu(OH)}_2\cdot\text{H}_2\text{O}(s) \longrightarrow 2\text{CuO}(s) + \text{CO}_2(g) + 2\text{H}_2\text{O}(g)$$
You are to determine the change in mass as the solid is heated and decomposed.
(a) Method – Read through the instructions before starting any practical work.
- Record all weighings in an appropriate form in the space below.
- Weigh and record the mass of an empty boiling-tube.
- Tip the contents of the tube labelled FA 3 into the weighed boiling-tube. Reweigh and record the total mass of the boiling-tube and FA 3.
- Heat FA 3 in the boiling-tube very gently until the vigorous decomposition of the copper carbonate has stopped; then heat more strongly for 1 to 2 minutes. Take care not to lose any solid from the tube during the initial heating.
- Warm the upper parts of the boiling-tube to evaporate any water that may have condensed while heating the carbonate.
- Place the hot tube on a heat-proof mat and leave to cool.
You are advised to continue with part (d) of this question or to start another question while the tube cools. - When cool, reweigh the boiling-tube and the residual copper(II) oxide.
- Reheat, cool and reweigh the tube until you are satisfied decomposition is complete.
Results
In an appropriate form, in the space below, record all of your balance readings, the mass of basic copper(II) carbonate and the mass of residual copper oxide. [6 marks]
(b) Calculate the loss in mass during the experiment as a percentage of the mass of solid heated. [1 mark]
(c) The theoretical loss in mass is 33.5%.
The proportions of $\text{CuCO}_3$ and $\text{Cu(OH)}_2$ in the basic carbonate can vary from the 1:1 ratio given in the formula.
Make use of the following information to account for the difference between the value you have calculated in (b) and the theoretical percentage loss in mass.
[Image_1:
| 1 mol $\text{CuCO}_3(s) \longrightarrow 1$ mol $\text{CO}_2(g)$ |
| 1 mol $\text{Cu(OH)}_2(s) \longrightarrow 1$ mol $\text{H}_2\text{O}(g)$ |
| Assume that 1 mol of any sample of the solid basic carbonate contains 1 mol $\text{H}_2\text{O}$. |
| $[M_r: \text{CO}_2, 44.0; \text{H}_2\text{O}, 18.0]$ |
.................................................................................................................................
.................................................................................................................................
................................................................................................................................. [1 mark]
(d) Add to the diagram below additional standard laboratory apparatus that would enable you to collect and measure the volume of carbon dioxide evolved in the experiment. Ensure that your apparatus does not also collect and measure any of the water vapour evolved.
[Image_2:
[2 marks]
555
528
558
(a) Carry out the following tests. Record your observations in the spaces provided in the table.
You should rinse and reuse test-tubes where possible.
| | observations |
|--------|---------------------|
| test | FA 4 | FA 5 | FA 6 | FA 7 |
| (i) | To 1 cm depth of solution in a test-tube add 1 cm depth of aqueous sodium hydroxide. Swirl the tube, then add a further 2 cm depth of aqueous sodium hydroxide. | | | |
| (ii) | To 1 cm depth of solution in a test-tube add 1 cm depth of aqueous ammonia. Swirl the tube, then add a further 2 cm depth of aqueous ammonia. | | | |
| (iii) | To 1 cm depth of solution in a test-tube add 1 cm depth of aqueous potassium iodide. | | | |
(b) Use the Qualitative Analysis Notes on page 11 to identify the cation present in each of the solutions. Complete the table below to identify each ion and to give supporting evidence from your observations.
| solution | cation | supporting evidence |
|----------|--------|---------------------|
| FA 4 | | |
| FA 5 | | |
| FA 6 | | |
| FA 7 | | |
(c) Use the Qualitative Analysis Notes on pages 11 and 12 to select a further reagent that could be used to confirm the presence of $Pb^{2+}$ in one of the solutions FA 4, FA 5, FA 6 and FA 7.
The reagent is .................................................... .
(d) FA 8 contains aqueous copper(II) ions. Carry out the following tests and make careful observations of all that happens in each experiment.
| | observations |
|--------|---------------------|
| test | |
| (i) | To 2 cm depth of FA 8 in a boiling-tube add 1 spatula measure of zinc metal powder. Leave to stand for 1 minute, then add 2 cm depth of distilled water and leave to stand for a further 2 minutes.|
| (ii) | To 1 cm depth of FA 8 in a test-tube add 1 cm depth of concentrated hydrochloric acid. (Care: corrosive) Retain the mixture for use in (iii). |
| (iii) | Using a dropping pipette transfer 1 cm depth of the solution in (ii) into another test-tube. Add 1 cm depth of water and shake the tube. |
(e) From your observations in (d)(i) complete the equation below:
$Cu^{2+}(aq) + Zn(s) \rightarrow$
600
595
508
