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(a) Method
Preparing a solution of FA 2
• Weigh the stoppered container of FA 2. Record the mass in the space below.
• Tip all the FA 2 into the beaker.
• Reweigh the container with its stopper. Record the mass.
• Calculate and record the mass of FA 2 used.
• Add approximately 100 cm³ of distilled water to the FA 2 in the beaker.
• Stir the mixture with a glass rod until all the FA 2 has dissolved.
• Transfer this solution into the 250 cm³ volumetric flask.
• Wash the beaker with distilled water and transfer the washings to the volumetric flask.
• Rinse the glass rod with distilled water and transfer the washings to the volumetric flask.
• Make up the solution in the volumetric flask to the mark using distilled water.
• Shake the flask thoroughly.
• This solution of impure NaHCO₃ is FA 3. Label the flask FA 3.
Results
Titration
• Fill the burette with FA 1.
• Pipette 25.0 cm³ of FA 3 into a conical flask.
• Add several drops of methyl orange.
• 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 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 1 added in each accurate titration.
Keep FA 1 for use in Question 2.
(b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value.
25.0 cm³ of FA 3 required ........................... cm³ of FA 1. [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 sulfuric acid present in the volume of FA 1 calculated in (b).
moles of $H_2SO_4$ = ................................ mol
(ii) Balance the equation for the reaction of sulfuric acid and sodium hydrogencarbonate. State symbols are not required.
$........NaHCO_3 + ........H_2SO_4 \rightarrow ........Na_2SO_4 + ........CO_2 + ........H_2O$
(iii) Using your answers to (i) and (ii), calculate the number of moles of sodium hydrogencarbonate used in each titration.
moles of $NaHCO_3$ = .................................. mol
(iv) Using your answer to (iii), calculate the mass of sodium hydrogencarbonate present in the mass of FA 2 used to prepare FA 3.
mass of $NaHCO_3$ = ................................... g
(v) Calculate the percentage purity by mass of the impure sodium hydrogencarbonate sample, FA 2.
percentage purity by mass of impure $NaHCO_3$, FA 2 = ................................. %
(vi) What did you assume about the impurities in FA 2 when you calculated the percentage purity?
...............................................................
...............................................................
(vii) A volumetric flask was labelled 250.0 ± 0.10 cm³.
Calculate the maximum percentage error when using this volumetric flask.
maximum percentage error = .............................. %
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592
532
When baking soda is heated, carbon dioxide is produced. In this experiment you will investigate the reaction taking place when the sodium hydrogencarbonate in baking soda is thermally decomposed. FA 4 is baking soda (impure NaHCO₃). Its composition is the same as that of FA 2. (a) Method
Record all your readings in the space below.
• Weigh the crucible with its lid.
• Transfer all the FA 4 from the container into the crucible.
• Weigh the crucible, lid and FA 4.
• Calculate and record the mass of FA 4 used.
• Place the crucible and contents on a pipe-clay triangle.
• Heat gently, with the lid on, for approximately one minute.
• Heat strongly, with the lid off, for a further three minutes.
• Replace the lid and leave the crucible to cool for at least five minutes.
While the crucible is cooling you may wish to begin work on Question 3.
• When it is cool, weigh the crucible with its lid and contents.
• Heat strongly, with the lid off, for a further two minutes.
• Replace the lid and leave the crucible to cool for at least five minutes.
• When it is cool, weigh the crucible with its lid and contents.
• Calculate and record the mass of residue obtained.
• This residue is FA 5. Keep this for use in 2(d).
Results
(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Use the percentage purity by mass of FA 2 you calculated in 1(c)(v), to calculate the mass of sodium hydrogencarbonate in the sample of FA 4 that you weighed out.
(If you were unable to carry out the calculation in 1(c)(v), assume that the percentage purity by mass of FA 2 is 95.8%.)
mass of NaHCO₃ in FA 4 weighed out = .............................. g
(ii) Calculate the mass of impurity present in your sample of FA 4.
mass of impurity = .............................. g
(iii) The impurity in FA 4 does not decompose when it is heated. This means that the residue, FA 5, contains the mass of impurity calculated in (ii) together with the solid decomposition product of sodium hydrogencarbonate.
Calculate the mass of the solid decomposition product.
mass of solid decomposition product = .............................. g
(iv) Use your answers to (i) and (iii) to calculate the mass of solid decomposition product that would be obtained if 84.0 g of pure sodium hydrogencarbonate were heated.
mass of solid decomposition product = .............................. g
(v) A student carried out the experiment by heating to constant mass and calculated that heating 84.0 g of pure NaHCO₃ would produce 52.3 g of the solid decomposition product. The student then suggested the following equation for the thermal decomposition of sodium hydrogencarbonate.
$$\text{NaHCO}_3(s) \rightarrow \text{NaOH}(s) + \text{CO}_2(g)$$
Use data from the Periodic Table on page 12 to explain why the student's suggestion cannot be correct.
(c) (i) Why was the lid put on while the crucible and its contents cooled?
(c) (ii) The experiment could be made more accurate by heating to constant mass or using a more accurate balance. Suggest a further improvement to make the experiment more accurate.
(d) (i) Pour a 1 cm depth of sulfuric acid, FA 1, into a test-tube. Add some FA 5 from the crucible to the acid in the test-tube. Record all your observations.
(d) (ii) Use your observation(s) in (i) to identify an anion present in FA 5. Explain your answer.
identity .................................................
explanation .................................................
(d) (iii) Steam is one of three products obtained when sodium hydrogencarbonate is thermally decomposed. Use your answer in (ii) to complete and balance the equation for the thermal decomposition of sodium hydrogencarbonate. Include state symbols.
.........NaHCO₃(s) → ........H₂O(g) + ........CO₂(g) + ..............................
(d) (iv) State whether the balanced equation in (iii) agrees with the student's results given in 2(b)(v). Show working in order to explain your answer.
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537
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Qualitative Analysis
At each stage of any test you are to record details of the following.
- colour changes seen
- the formation of any precipitate
- the solubility of such precipitates in an excess of the reagent added
Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given.
Where gases are released they should be identified by a test, described in the appropriate place in your observations.
You should indicate clearly at what stage in a test a change occurs. No additional tests for ions present should be attempted.
If any solution is warmed, a boiling tube MUST be used. Rinse and reuse test-tubes and boiling tubes where possible.
(a) (i) $FA\ 6$ and $FA\ 7$ are aqueous solutions.
Each solution contains one cation and one anion from those listed in the Qualitative Analysis Notes.
Use 1 cm depths of $FA\ 6$ or $FA\ 7$ in test-tubes for the following tests.
Complete the table by recording your observations.
[Table_1: Table for recording observations for FA 6 and FA 7 under different tests]
Add a few drops of aqueous barium chloride or aqueous barium nitrate, then add dilute nitric acid.
Add a few drops of aqueous silver nitrate.
Add a small spatula measure of sodium carbonate. Shake the mixture.
(ii) From your observations, deduce which solution, $FA\ 6$ or $FA\ 7$, has the lower pH. Give your evidence.
solution with lower pH .....................
evidence ............................................................................................................................
............................................................................................................................ [4]
(b) Choose two reagents that would allow you to identify the cations in $FA\ 6$ and $FA\ 7$.
reagents ............................................................ and ............................................................
Use these reagents to test solutions $FA\ 6$ and $FA\ 7$. Record all your observations in the space below.
[4]
(c) Deduce the chemical formulae of $FA\ 6$ and $FA\ 7$.
$FA\ 6$ ........................................................
$FA\ 7$ ........................................................ [2]
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542
