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You must prepare Flask A and Flask B in Question 2 before starting Question 1. Shake each flask periodically during the time you spend on Question 1.
FB 1 is 0.125 mol dm-3 sulfuric acid, $H_2SO_4$.
FB 2 is an aqueous solution of sodium hydroxide, NaOH.
You are to determine the concentration, in mol dm-3, of the sodium hydroxide in FB 2.
(a) Method
- Fill a burette with FB 1.
- Run between 45.50 cm3 and 46.50 cm3 of FB 1 from the burette into the 250 cm3 graduated (volumetric) flask, labelled FB 3.
- Make up to the mark with distilled water.
- Shake the flask to mix the solution.
You are reminded to shake Flask A and Flask B periodically.
Titration
- Fill a second burette with FB 2.
- Pipette 25.0 cm3 of FB 3, the diluted acid, into a conical flask.
- Add to the flask a few drops of phenolphthalein indicator.
- Place the flask on a white tile.
- Titrate the acid in the flask with FB 2.
Note: The “permanent” pink colour will fade over several minutes as carbon dioxide is absorbed from the atmosphere.
You should perform a rough titration.
In the space below record your burette readings for this rough titration.
The rough titre is ....................... cm3.
- Carry out as many accurate titrations as you think necessary to obtain consistent results.
- Record in a suitable form on page 3 all of your burette readings and the volume of FB 2 added in each accurate titration.
- Make certain any recorded results show the precision of your practical work.
(b) From your accurate titration results obtain a suitable value to be used in your calculation. Show clearly how you have obtained this value. [7]
25.0 cm3 of FB 3 required ....................... cm3 of FB 2. [1]
Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(c) (i) Calculate how many moles of $H_2SO_4$ in FB 1 were run from the burette into the 250 cm3 graduated, (volumetric) flask.
....................... mol of $H_2SO_4$ were run from the burette into the graduated flask.
(ii) Calculate how many moles of $H_2SO_4$ in FB 3 were pipetted from the graduated flask into the conical flask in each titration.
....................... mol of $H_2SO_4$ were pipetted into the conical flask.
(iii) Calculate how many moles of NaOH reacted with the $H_2SO_4$ in (ii).
$H_2SO_4 (aq) + 2NaOH (aq) \rightarrow Na_2SO_4 (aq) + 2H_2O (l)$
The $H_2SO_4$ in the titration flask reacted with ....................... mol of NaOH.
(iv) Calculate the concentration, in mol dm-3, of NaOH in FB 2.
The concentration of NaOH in FB 2 is ....................... mol dm-3. [5]
(d) The individual error in any burette reading is ±0.05 cm3.
Two students, A and B, record identical burette readings.
final burette reading 25.60 cm3
initial burette reading 1.35 cm3
volume added 24.25 cm3
Explain the following.
(i) The initial burette reading made by student A was 0.05 cm3 greater than the true value but the volume added was exactly 24.25 cm3.
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(ii) The initial burette reading made by student B was 0.05 cm3 less than the true value and the actual volume added was exactly 24.15 cm3.
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[2]
(e) In the instructions for the experiment you were told that the “permanent” pink colour at the end-point would fade over a few minutes as carbon dioxide is absorbed from the atmosphere.
(i) Explain why absorption of carbon dioxide at the end-point would reverse the indicator colour change seen in the titration.
..........................................................................................................................
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(ii) Suggest a modification to the titration method, using the same indicator, that would overcome this problem.
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[2]
588
510
535
Results
Record, in a single table below, the burette readings and volume of FB 4 added, for each of Flask A and Flask B.
(b) Calculations
In these calculations make use of the following.
- The concentration of NaOH in FB 4 is 0.050moldm−3.
- 50cm3 of 0.200moldm−3 propanoic acid, the volume of acid added to each flask, contains 0.010 mol C2H5CO2H.
- 1 mol C2H5CO2H reacts with 1 mol NaOH.
Volume of FB 4 = ............................. cm3
(ii) For each flask, use your titration result in (a) to calculate the volume of FB 4 needed to react with the acid remaining in 50cm3 of the aqueous layer, after shaking with the organic liquid.
| Flask A | Flask B |
|---|---|
| volume of FB 4 = ......................... cm3 | volume of FB 4 = ......................... cm3 |
(answer to (i) − answer to (ii))
For each flask evaluate this expression.
Flask A (answer to (i) − answer to (ii)) = .................................. cm3
Flask B (answer to (i) − answer to (ii)) = .................................. cm3
(c) In which flask was most propanoic acid transferred to the organic layer?
Justify your answer.
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(d) It is suggested that shaking the mixture leads to the following equilibrium being established.
C2H5CO2H(aq) ⟷ C2H5CO2H(org)
Determine the equilibrium constant by evaluating the expressions in the following table. Ignore units.
(i) Determine the equilibrium constant by evaluating the expressions in the following table. Ignore units.
| Flask A | Flask B |
|---|---|
| Kc = \(\frac{\text{answer (b)(iii)} \times 1.25}{\text{answer (b)(ii)}}\) | Kc = \(\frac{\text{answer (b)(iii)} \times 0.83}{\text{answer (b)(ii)}}\) |
| Kc = ..................... | Kc = ..................... |
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.............................................................................................................................................
557
591
521
(a) (i) One or more of the solutions $\text{FB 7}$, $\text{FB 8}$ and $\text{FB 9}$ are believed to contain the ammonium ion, $\text{NH}_4^+$.
Suggest a reagent that would enable you to identify the presence of $\text{NH}_4^+$ and describe how you would use the reagent in an appropriate test.
reagent ............................................................................................................................
test .................................................................................................................................
.................................................................................................................................
Use this reagent to test each of the solutions. Record your observations in the table below.
| solution | observation |
|----------|-------------|
| FB 7 | |
| FB 8 | |
| FB 9 | |
(ii) One or more of the solutions contains the sulfate ion, $\text{SO}_4^{2-}$.
Select reagents that would enable you to identify the presence of $\text{SO}_4^{2-}$.
Show clearly, by describing how the reagents will be used, how you would distinguish $\text{SO}_4^{2-}$ from the sulfite ion, $\text{SO}_3^{2-}$.
reagents .......................................................................................................................
test ..............................................................................................................................
Use these reagents to test each of the solutions. Record your observations in the table below.
| solution | observation |
|----------|-------------|
| FB 7 | |
| FB 8 | |
| FB 9 | |
(iii) Conclusions
The ammonium ion, $\text{NH}_4^+$, is present in ..................................................
The sulfate ion, $\text{SO}_4^{2-}$, is present in ......................................................
[5]
(b) Use aqueous sodium hydroxide and aqueous ammonia in separate tests to identify any cation (apart from $\text{NH}_4^+$) present in $\text{FB 7}$, $\text{FB 8}$ and $\text{FB 9}$.
Present your results for each of the solutions in a suitable form below.
[4]
(c) Conclusion
Complete the following table.
Place a cross in any box where no cation has been identified.
| solution | cation | supporting evidence |
|----------|--------|---------------------|
| FB 7 | | |
| FB 8 | | |
| FB 9 | | |
[1]
(d) Carry out the following tests on $\text{FB 10}$.
Observe carefully at each stage and record all of your observations in the table.
| test | observations |
|------|--------------|
| (i) | Place 2 spatula measures of $\text{FB 10}$ in a dry, hard glass boiling-tube.
Heat the solid gently at first, then strongly until no further change is seen.
Retain the solid for use in (ii). |
| (ii) | Tip the contents of the tube in (i) into a second boiling-tube.
Add 2 cm depth of dilute hydrochloric acid a little at a time.
Warm the tube and leave to stand. |
[5]
595
568
507
