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Acids are defined as substances that can donate hydrogen ions, H\(^+\), to bases. Monoprotic acids contain one H\(^+\) that can be donated per molecule. Diprotic acids contain two H\(^+\) that can be donated per molecule.
You will determine by a titration method whether acid \(Z\) is monoprotic or diprotic.
FA 1 is a solution containing 6.10 g dm\(^{-3}\) of acid \(Z\).
FA 2 is 0.105 mol dm\(^{-3}\) aqueous sodium hydroxide, NaOH. bromophenol blue indicator
(a) Method
- Pipette 25.0 cm\(^3\) of FA 1 into a conical flask.
- Fill a burette with FA 2.
- Add several drops of bromophenol blue indicator to the conical flask.
- Carry out a rough titration and record your burette readings 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 accuracy of your practical work.
- Record, in a suitable form below, all of your burette readings and the volume of FA 2 added in each accurate titration.
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(b) From your accurate titration results, obtain a suitable value for the volume of FA 2 to be used in your calculations. Show clearly how you obtained this value.
25.0 cm\(^3\) of FA 1 required ............................ cm\(^3\) of FA 2. [1]
(c) Calculations
- Calculate the number of moles of sodium hydroxide present in the volume of FA 2 calculated in (b).
moles of NaOH = ........................... mol
Then deduce the number of moles of H\(^+\) present in 25.0 cm\(^3\) of FA 1.
moles of H\(^+\) in 25.0 cm\(^3\) of FA 1 = ........................... mol [1] - Calculate the number of moles of H\(^+\) present in 1 dm\(^3\) of FA 1.
moles of H\(^+\) in 1 dm\(^3\) of FA 1 = ........................... mol [1] - FA 1 contains 6.10 g dm\(^{-3}\) of acid \(Z\). The relative molecular mass of \(Z\) is 126.
Calculate the number of moles of \(Z\) in 1 dm\(^3\) of FA 1.
moles of \(Z\) in 1 dm\(^3\) of FA 1 = ........................... mol [1] - Use your answers to (ii) and (iii) to determine whether \(Z\) is a monoprotic or a diprotic acid. Explain your answer.
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569
When an acid reacts with an alkali the neutralisation reaction is always exothermic.
$$\text{H}^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{H}_2\text{O}(l)$$
You will determine the enthalpy change of neutralisation, $\Delta H$, for a monoprotic acid X.
FA 3 is aqueous sodium hydroxide, NaOH.
FA 4 is a 2.00 mol dm$^{-3}$ solution of monoprotic acid X.
(a) Method
- Support the plastic cup in the 250 cm$^{3}$ beaker.
- Fill the second burette with FA 3.
- Use the measuring cylinder to transfer 25.0 cm$^{3}$ of FA 4 into the plastic cup.
- Measure and record the temperature of FA 4 in the plastic cup.
- Add 5.00 cm$^{3}$ of FA 3 from the burette into the plastic cup. Stir the contents of the cup. Read and record the maximum temperature of the solution.
- Add a further 5.00 cm$^{3}$ of FA 3 from the burette into the cup. Stir the contents of the cup. Read and record the maximum temperature of the solution.
- Repeat the addition of FA 3, in 5.00 cm$^{3}$ portions, until 50.00 cm$^{3}$ have been added. Read and record the maximum temperature of the solution after each addition.
[4 marks]
(b) On the grid plot a graph of temperature, (y-axis), against volume of FA 3 added, (x-axis). Your scale should allow a temperature of 2 $^{\circ}$C above the maximum measured to be plotted.
On your graph draw two lines of best fit. One line should be for when the temperature was rising and the other for after the maximum temperature had been reached. You should indicate clearly any points you consider to be anomalous.
Extrapolate the lines so that they intersect.
[4 marks]
(c) From your graph, determine the maximum temperature reached in the experiment and the volume of FA 3 added to produce this maximum temperature.
maximum temperature reached = .............................. $^{\circ}$C
volume of FA 3 added to reach maximum temperature = .............................. cm$^{3}$
[1 mark]
(d)
(i) Calculate the energy released during this experiment.
[Assume that 4.2 J of heat energy changes the temperature of 1.0 cm$^{3}$ of solution by 1.0$^{\circ}$C.]
energy released = .............................. J
[1 mark]
(ii) Calculate the number of moles of acid X in 25.0 cm$^{3}$ of FA 4.
moles of X = .............................. mol
[1 mark]
(iii) Calculate, in kJ mol$^{-1}$, the enthalpy change of neutralisation for acid X.
enthalpy change = ...... .............................. kJ mol$^{-1}$
[1 mark]
(e) Without changing the apparatus or solutions used, suggest one way in which the experiment could be modified to make the values obtained in (c) more accurate.
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[1 mark]
(f) The enthalpy change of neutralisation of hydrochloric acid with aqueous sodium hydroxide is more exothermic than the enthalpy change of neutralisation of acid X.
Explain what this tells you about acid X.
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[2 marks]
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(a) FA 5, FA 6 and FA 7 are $1.0 \text{mol dm}^{-3}$ sulfuric acid, $0.1 \text{mol dm}^{-3}$ sulfuric acid and $1.0 \text{mol dm}^{-3}$ hydrochloric acid but not necessarily in that order.
You are to plan and carry out tests to determine the identities of FA 5, FA 6 and FA 7. You should record, in a suitable form in the space below, your tests and observations. You should show clearly the observations for each of FA 5, FA 6 and FA 7 with all test reagents.
FA 5 is ................................................................. .
FA 6 is ................................................................. .
FA 7 is ................................................................. .
(b) FA 8 and FA 9 each contain one anion and one cation.
(i) Carry out the following tests and record your observations.
For each test you should use a small spatula measure of FA 8 or FA 9.
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| test | observations |
|:-----------------------------------------------------------------------------------------|:-----------------------------------------:|
| | FA 8 | FA 9 |
| Add a small spatula measure to a 1 cm depth of distilled water in a test-tube and shake. | | |
| Add a few drops of universal indicator. Record the pH of the mixture. | | |
| Heat a small spatula measure in a hard-glass test-tube. | | |
| Dissolve a small spatula measure in 2 cm depth of dilute hydrochloric acid in a boiling | | |
| tube (heat gently if necessary). | | |
| Place a 1 cm depth of this solution into two test-tubes. | | |
| To one test-tube, add aqueous sodium hydroxide. | | |
| To the second test-tube, add aqueous ammonia. | | |
(ii) Mix a spatula measure of FA 8 with a spatula measure of FA 9. Heat the mixture in a hard-glass test-tube. Record your observations.
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(iii) From your observations in (b)(i) and (b)(ii) identify the ions present in FA 8 and FA 9. If you are unable to identify an ion, write ‘unknown’.
FA 8 cation ..................................... anion .....................................
FA 9 cation ..................................... anion .....................................
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