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In this experiment you will determine the percentage by mass of an impure sample of sodium hydrogencarbonate, NaHCO_3. You will do this by titration with hydrochloric acid, HCl. The impurity in the sample is X. X is a sodium compound which does not react with HCl.
FB 1 is a mixture containing sodium hydrogencarbonate and X. You are supplied with approximately 6.5g of FB 1. You will also use FB 1 in Question 2. FB 2 is 0.105 mol dm^-3 hydrochloric acid, HCl. methyl orange indicator
(a) Method
Preparing a solution of FB 1
• Weigh the 100cm^3 beaker. Record the mass.
• Add between 2.8g and 3.0g of FB 1 to the beaker.
• Reweigh the beaker with FB 1. Record the mass.
• Calculate and record the mass of FB 1 used.
• Add approximately 50cm^3 of distilled water to FB 1 in the beaker.
• Stir the mixture with a glass rod until all the FB 1 has dissolved.
• Transfer this solution into the 250cm^3 volumetric flask.
• Wash the beaker with distilled water and transfer the washings to the volumetric flask.
• Add distilled water to the volumetric flask up to the mark.
• Shake the flask thoroughly.
• This solution of impure sodium hydrogencarbonate is FB 3. Label the flask FB 3.
Titration of FB 3
• Fill the burette with FB 2.
• Pipette 25.0 cm^3 of FB 3 into a conical flask.
• Add approximately 10 drops of methyl orange indicator.
• Carry out a rough titration.
• Record your burette readings and the rough titre in the space below.
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 FB 2 added in each accurate titration.
[8]
(b) From your accurate titration results, obtain a suitable value for the volume of FB 2 to be used in your calculations. Show clearly how you obtained this value.
25.0 cm^3 of FB 3 required ............................ cm^3 of FB 2. [1]
(c) Calculations
(i) Give your answers to (ii), (iii), (iv) and (v) to the appropriate number of significant figures. [1]
(ii) Calculate the number of moles of hydrochloric acid, HCl, in the volume of FB 2 calculated in (b).
moles of HCl = .................................. mol [1]
(iii) Complete and balance the equation for the reaction of sodium hydrogencarbonate with hydrochloric acid. Include state symbols.
....NaHCO_3...... + ....HCl..... → ....NaCl..... + ....CO_2...... + .................
Deduce the number of moles of sodium hydrogencarbonate that reacted with the number of moles of HCl calculated in (ii).
moles of NaHCO_3 = ................................. mol [1]
(iv) Use your answer to (iii) to calculate the number of moles of sodium hydrogencarbonate in the FB 1 that you weighed out.
moles of NaHCO_3 in FB 1 used = ................................. mol [1]
(v) Calculate the percentage by mass of NaHCO_3 in FB 1.
percentage by mass of NaHCO_3 in FB 1 = ................................. % [1]
551
595
510
You will determine the percentage by mass of NaHCO_3 in FB 1 again, this time by thermal decomposition.
The equation for the thermal decomposition of sodium hydrogencarbonate is shown.
$2\text{NaHCO}_3(s) \rightarrow \text{Na}_2\text{CO}_3(s) + \text{CO}_2(g) + \text{H}_2\text{O}(g)$
FB 1 is a mixture containing sodium hydrogencarbonate and an impurity, X.
(a) Method
• Weigh a crucible with its lid and record the mass.
• Add between 2.8 g and 3.0 g of FB 1 to the crucible. Weigh the crucible and lid with FB 1 and record the mass.
• Place the crucible on the pipe-clay triangle. Heat the crucible and contents gently for approximately two minutes, with the lid off.
• Then heat strongly for approximately three minutes.
• Replace the lid and leave the crucible and residue to cool for at least five minutes.
While the crucible is cooling, you may wish to begin work on Question 3.
• Reweigh the crucible and contents with the lid on. Record the mass.
• Heat the crucible and contents strongly for a further two minutes, without the lid.
• Replace the lid and leave the crucible and residue to cool for at least five minutes.
• Reweigh the crucible and contents with the lid on. Record the mass.
• Calculate and record the starting mass of FB 1 and the mass of residue obtained.
[5]
(b) Calculations
(i) Calculate the number of moles of carbon dioxide produced during the thermal decomposition of FB 1 by using the formula below.
$$\text{moles of CO}_2 \text{ produced} = \frac{\text{mass lost during heating}}{M_{\text{CO}_2} + M_{\text{H}_2\text{O}}}$$
moles of CO_2 produced = ............................................. mol [1]
(ii) Use your answer to (i) and the equation on page 6 to calculate the mass of sodium hydrogencarbonate in the FB 1 you used in this experiment.
mass of NaHCO_3 in FB 1 = ............................................. g [1]
(iii) For this experiment calculate the percentage by mass of NaHCO_3 in FB 1.
percentage by mass of NaHCO_3 in FB 1 = ............................................. % [1]
(c) (i) Explain why the crucible and contents were heated for a further two minutes after the first weighing of the crucible and residue.
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[2]
(ii) What assumption did you make about the behaviour of X when you carried out the calculation?
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[1]
(iii) A student suggested that it would have been more accurate to carry out the thermal decomposition with the lid on the crucible throughout the experiment.
State and explain whether or not you agree with this suggestion.
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[1]
(iv) Suggest which of the two procedures, titration or thermal decomposition, gives a more accurate value for the percentage by mass of NaHCO_3 in FB 1. Explain your choice.
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[1]
543
592
576
(a) FB 4 has the same composition as the residue obtained in Question 2. It contains two sodium compounds, one of which is X. Both anions present in FB 4 are listed in the Qualitative Analysis Notes.
(i) To a small spatula measure of FB 4 in a test-tube, add dilute nitric acid slowly. Record your observations. Keep the solution produced for use in (ii).
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(ii) To a 1 cm depth of the solution obtained in (i) in a test-tube, add a few drops of aqueous silver nitrate, followed by aqueous ammonia. Record your observations.
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(iii) Give the equation for one reaction taking place in (i). State symbols are not required.
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(b) Dissolve the remaining FB 4 in a 5 cm depth of distilled water in a boiling tube. This solution is FB 5.
(i) Carry out the following tests and record your observations. Use a 1 cm depth of FB 5 in a test-tube for each test.
[Table]
test | observations
--- | ---
Add several drops of aqueous copper(II) sulfate, then
add dilute sulfuric acid.
Add a few drops of aqueous barium chloride or aqueous barium nitrate, then
add dilute nitric acid.
Add a few drops of methyl orange indicator.
Add several drops of aqueous silver nitrate, then
add dilute nitric acid.
[6]
(ii) To a 1 cm depth of FB 5 in a boiling tube, add an equal volume of aqueous sodium hydroxide and warm carefully, then add a small piece of aluminium foil to the mixture. Record your observations.
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(iii) Using your observations in (a) and (b), name X.
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(iv) What can you deduce about FB 4 from the observation when methyl orange indicator was added to FB 5?
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526
547
579
