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In this question you will determine the percentage purity of a sample of contaminated sodium carbonate. FB 1 is a solution of the contaminated sodium carbonate. You will first dilute FB 1 and then titrate the diluted solution using hydrochloric acid.
$$2HCl(aq) + Na_2CO_3(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)$$
FB 1 was prepared by dissolving 125 g of contaminated sodium carbonate, $Na_2CO_3$, in distilled water and making the solution up to 1 dm³. FB 2 is 0.100 mol dm⁻³ hydrochloric acid, $HCl$.
methyl orange indicator
(a) Method
**Dilution**
• Fill the burette with FB 1.
• Run between 13.00 and 13.50 cm³ of FB 1 into the 250 cm³ volumetric (graduated) flask. Record the volume in the space below.
volume of FB 1 = .................... cm³
• Fill the volumetric flask to the line with distilled water. Stopper the flask and shake it to ensure thorough mixing.
• Label this flask FB 3.
**Titration**
• Rinse the burette thoroughly with distilled water and then with a little of solution FB 2.
• Fill the burette with FB 2.
• Use the pipette to transfer 25.0 cm³ of FB 3 into a conical flask.
• Add 5-10 drops of methyl orange indicator.
• Perform a rough titration and record your burette readings in the space below.
The rough titre = .................... cm³.
• Do 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.
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value.
25.0 cm³ of FB 3 required .................... cm³ of FB 2. [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 hydrochloric acid present in the volume of FB 2 calculated in (b).
moles of $HCl$ = .................... mol
(ii) Calculate the number of moles of sodium carbonate present in 25.0 cm³ of FB 3.
moles of $Na_2CO_3$ = .................... mol
(iii) Calculate the concentration, in mol dm⁻³, of sodium carbonate in FB 3.
concentration of $Na_2CO_3$ in FB 3 = .................... mol dm⁻³
(iv) Calculate the concentration, in mol dm⁻³, of sodium carbonate in FB 1.
concentration of $Na_2CO_3$ in FB 1 = .................... mol dm⁻³
(v) Calculate the percentage purity by mass of the sodium carbonate in the contaminated sample used to prepare solution FB 1.
$A_r$: C, 12.0; O, 16.0; Na, 23.0
percentage purity by mass = .................... % [5]
549
584
537
In this experiment you will determine the percentage by mass of magnesium in a mixture of magnesium and iron. To do this you will measure the heat given out by the reaction of the mixture with excess hydrochloric acid. You should assume that only the magnesium in the mixture will react with the acid.
FB 4 is the mixture of magnesium and iron.
FB 5 is 2.00 mol dm-3 hydrochloric acid, $HCl$.
(a) Method
Read through the instructions carefully and prepare a table below for your results before starting any practical work.
• Support the plastic cup in a 250 cm3 beaker.
• Use the measuring cylinder to transfer 40 cm3 of FB 5 into the plastic cup.
• Weigh the stoppered container with the FB 4 and record the mass.
• Measure and record the initial temperature of the solution in the plastic cup.
• Add all the sample of FB 4 to the acid in the plastic cup carefully to minimise acid spray.
• Stir the mixture and record the maximum temperature that is reached.
• Weigh the stoppered container and any residual metal mixture. Record the mass.
• Record the mass of metal added to the acid.
(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Use your value for the maximum temperature rise to calculate the heat energy produced in the reaction.
(Assume that 4.2 J are required to increase the temperature of 1.0 cm3 of solution by 1.0°C.)
heat energy produced = J
(ii) The molar enthalpy change, $\Delta H$, for the reaction shown below is $-457 \text{kJ mol}^{-1}$.
$$ \text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)} $$
Use this value and your answer to (i) to calculate the number of moles of magnesium in your reaction.
moles of Mg = mol
(iii) Use your answer to (ii), to calculate the percentage by mass of magnesium in FB 4. ($A_r$: Mg, 24.3)
percentage by mass of Mg in FB 4 = %
(iv) You have assumed that only the magnesium reacts with the acid. What other assumption have you made in using this method to determine the percentage by mass of magnesium in the mixture?
..........................................................................................................................
..........................................................................................................................
(c) (i) Calculate the maximum percentage error in the temperature rise.
maximum percentage error = %
(ii) Using the same FB 4, the same FB 5 and no additional apparatus, describe how you could modify the experiment to reduce this maximum percentage error in the temperature rise. Explain your answer.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
511
570
580
(a) FB 6 is a solution containing two cations and one anion. Both of the cations are listed in the Qualitative Analysis Notes on page 10.
(i) Select reagents to identify the cations in FB 6. Do tests using these reagents and record your results in the space below.
[Image of space for answers]
The cations in FB 6 are ............................................. and ............................................... .
(ii) The anion in FB 6 is the nitrate ion, NO₃⁻. Explain why it is difficult to confirm the presence of the nitrate ion in FB 6 using the method outlined in the Qualitative Analysis Notes on page 11.
[Image of space for answers]
[6]
(b) FB 7 is a halogenoalkane. Do the following test to determine the halogen present in FB 7.
[Table_1]
(i) To a 1 cm depth of ethanol in a test-tube, add a 1 cm depth of silver nitrate followed by 10 drops of FB 7, then place the test-tube in the hot water bath.
The halogen present in FB 7 is ......................................................................... .
(ii) What type of reaction has the halogenoalkane undergone?
[Image of space for answers]
[4]
(c) FB 8 and FB 9 each contain a single cation and a single anion. Do the following tests and record your observations.
[Table_2]
(i) To a 1 cm depth of FB 8 in a test-tube, add FB 9 until there is no further reaction, then add aqueous barium chloride or aqueous barium nitrate.
(ii) Suggest possible identities for the ions present. You will only be able to identify one of the cations.
cation ..................................................................................
anions ..................................................................................
[5]
528
541
511
