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In this experiment you will determine the oxidation number of iodine in one of its compounds by titration.
FA 1 is a 0.0197 mol dm$^{-3}$ solution of the iodine-containing compound.
FA 2 is dilute sulfuric acid, H$_2$SO$_4$.
FA 3 is aqueous potassium iodide, KI.
FA 4 is 0.105 mol dm$^{-3}$ sodium thiosulfate, Na$_2$S$_2$O$_3$.
starch indicator
FA 1 reacts with excess acidified potassium iodide to produce iodine, I$_2$. This iodine is then titrated with aqueous sodium thiosulfate using starch indicator.
(a) Method
- Fill the burette with FA 4.
- Pipette 25.0 cm$^3$ of FA 1 into a conical flask.
- Using the measuring cylinder, add 10 cm$^3$ of FA 2 to the same conical flask.
- Using the same measuring cylinder, add 20 cm$^3$ of FA 3 to the mixture in the conical flask. The mixture will now be a red-brown colour, due to iodine produced.
- Carry out a rough titration by adding FA 4 from the burette until the mixture becomes light brown.
- Then add 10 drops of starch indicator. The mixture will change to a dark blue colour.
- Continue titrating until the mixture becomes colourless. This is the end-point.
The rough titre is ............................ cm$^3$. - Carry out as many accurate titrations as you think necessary to obtain consistent results.
- Make sure 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 4 added in each accurate titration.
(b) From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value.
The iodine produced required ............................ cm$^3$ of FA 4. [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 sodium thiosulfate in the volume of FA 4 calculated in (b).
moles of Na$_2$S$_2$O$_3$ = ............................ mol
(ii) The equation for the reaction of iodine with sodium thiosulfate is shown.
$\mathrm{I_2(aq) + 2Na_2S_2O_3(aq) \rightarrow Na_2S_4O_6(aq) + 2NaI(aq)}$
Calculate the number of moles of iodine that reacted with the sodium thiosulfate calculated in (i).
moles of I$_2$ = ............................ mol
(iii) Use the information on page 2 to calculate the number of moles of iodine-containing compound in the 25 cm$^3$ of FA 1 used in each titration.
moles of iodine-containing compound in 25 cm$^3$ FA 1 = ............................ mol
(iv) Use your answers to (ii) and (iii) to calculate the number of moles of iodine produced when 1 mole of the iodine-containing compound in FA 1 reacts with excess FA 3. Give your answer as an integer.
moles of I$_2$ = ............................ mol
(v) The anion in FA 1 is IO$_x^{-}$ where x is the number of oxygen atoms present in the formula.
Use your answer to (iv) to balance the ionic equation for the reaction between FA 1 and FA 3 under acidic conditions.
Hence deduce the value of x in the formula IO$_x^{-}$.
IO$^{-} ...$ + ......I$^{-}$ + ......H$^+$ $\rightarrow$ ......I$_2$ + ......H$_2$O
x = ............................
(vi) Calculate the oxidation state of iodine in FA 1.
(If you were unable to calculate x in part (v), assume that x = 4.)
oxidation state of iodine = ............................
[6]
586
551
574
Zinc carbonate occurs in a basic form, which means that zinc hydroxide is also present. The chemical formula of basic zinc carbonate can be written as $ZnCO_3.yZn(OH)_2$, where $y$ may not be an integer. In this experiment you will heat basic zinc carbonate to decompose it and use your results to determine the value of $y$.
When basic zinc carbonate is heated, it decomposes as shown.
$ZnCO_3.yZn(OH)_2(s) \rightarrow (1+y)ZnO(s) + CO_2(g) + yH_2O(g)$
FA 5 is basic zinc carbonate, $ZnCO_3.yZn(OH)_2$.
(a) Method
Read through the method before starting any practical work.
Prepare a table for all your results from Experiments 1 and 2 in the space on page 5.
Experiment 1
• Weigh a crucible with its lid and record the mass.
• Add 2.1 − 2.5 g of FA 5 to the crucible. Weigh the crucible and lid with FA 5 and record the mass.
• Place the crucible in the pipe-clay triangle on top of the tripod.
• Heat the crucible and contents gently for 1 minute with the lid on.
• Remove the lid. Heat the crucible and contents strongly, with the lid off, for approximately 4 minutes.
• Replace the lid and leave the crucible and residue to cool for at least 5 minutes, before re-weighing it with the lid on. Record the mass.
• While the crucible is cooling, you may wish to begin work on Question 3.
• Calculate, and record in your table, the mass of FA 5 used and the mass of residue obtained.
(i) State the observation(s) you made while you were heating FA 5.
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(ii) State the observation(s) you made once the residue had cooled down.
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Experiment 2
• Repeat the procedure used in Experiment 1, using 1.5 − 1.9 g of FA 5 and using the other crucible and lid.
• Record the three balance readings made during the experiment.
• Calculate and record the mass of FA 5 used and the mass of residue obtained.
Results
(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate the relative formula mass, $M_r$, of zinc hydroxide, $Zn(OH)_2$.
$M_r$ of $Zn(OH)_2 = ext{..........................}$
(ii) Using your answer to (i), write down an expression, in terms of $y$, for the relative formula mass, $M_r$, of basic zinc carbonate, $ZnCO_3.yZn(OH)_2$.
$M_r$ of $ZnCO_3.yZn(OH)_2 = ext{............................................................}$
(iii) Using the mass of $ZnCO_3.yZn(OH)_2$ from Experiment 1 and your answer to (ii), write down an expression, in terms of $y$, for the number of moles of $ZnCO_3.yZn(OH)_2$ that you heated in Experiment 1.
moles of $ZnCO_3.yZn(OH)_2 = ext{............................................................} \text{mol}$
(iv) Using your answer to (iii) and the equation below, write an expression, in terms of $y$, for the number of moles of zinc oxide produced in Experiment 1.
$ZnCO_3.yZn(OH)_2(s) \rightarrow (1+y)ZnO(s) + CO_2(g) + yH_2O(g)$
moles of $ZnO$ produced = ext{...........................................................} \text{mol}$
(v) Use your results from Experiment 1 to calculate the number of moles of zinc oxide, $ZnO$, obtained in the residue. You may assume complete decomposition has occurred.
moles of $ZnO = ext{...........................................................} \text{mol}$
(vi) Using your answers to (iv) and (v), calculate the value of $y$ to one decimal place.
$y = ext{............................}$
(c) (i) Apart from altering the balance or the masses of FA 5 used, state one improvement you could make to the experimental procedure to improve its accuracy.
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(ii) Which experiment should be more accurate, Experiment 1 or Experiment 2? Explain your answer.
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539
544
511
FA 6, FA 7 and FA 8 are solutions of salts.
[Table_1]
(a) You will identify the cations present in FA 6, FA 7 and FA 8.
To do this you will carry out six separate tests. You will use dilute sulfuric acid and aqueous sodium hydroxide separately with FA 6, FA 7 and FA 8.
Use a 1 cm depth of each salt solution in a suitable tube for each test you carry out.
Record all of your observations in a table in the space below.
(b) Name the reagents you would use to identify the halide ion present in either FA 7 or FA 8. Test FA 7 and FA 8 with these reagents and record your observations.
reagents used ............................................................................................................................
[Table_2]
(c) (i) Name the reagents you would use to confirm the presence of the nitrogen-containing anions in the two solutions that do not contain a halide ion. Test both solutions with these reagents and record your observations.
reagents used ..............................................................................................................................
[Table_3]
(ii) Name the reagent you would use to positively identify one of the nitrogen-containing anions in the two solutions tested in (i). Test both solutions with this reagent. Record all your observations.
reagent used ...............................................................................................................................
[Table_4]
(d) Use the information given in (a) and your observations in all tests to deduce the chemical formulae of the three salts.
FA 6 is ......................................... FA 7 is ......................................... FA 8 is .........................................
590
539
503
