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HA is an organic acid where A⁻ is the anion. You will determine the relative formula mass, $M_r$, of HA by titration with sodium hydroxide of known concentration and so identify the anion, A⁻. The equation for the reaction is shown.
$$\text{HA(aq) + NaOH(aq) → NaA(aq) + H}_2\text{O(l)}$$
FB 1 is a solution of organic acid, HA, containing 12.60 g dm⁻³.
FB 2 is 0.100 mol dm⁻³ sodium hydroxide, NaOH.
thymol blue indicator
(a) Method
• Fill the burette with FB 1.
• Pipette 25.0 cm³ of FB 2 into a conical flask.
• Add approximately 10 drops of thymol blue indicator. This indicator is blue in alkaline solutions and yellow in acidic solutions.
• 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 certain that 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 1 added in each accurate titration.
(b) From your accurate titration results, obtain a suitable value for the volume of FB 1 to be used in your calculations. Show clearly how you obtained this value.
25.0 cm³ of FB 2 required .......................... cm³ of FB 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 sodium hydroxide present in 25.0 cm³ of FB 2 pipetted into the conical flask.
moles of NaOH = .............................. mol
(ii) Use your answers to (b) and (c)(i) to determine the concentration of the organic acid HA, in FB 1, in mol dm⁻³.
concentration of HA in FB 1 = .............................. mol dm⁻³
(iii) Use your answer to (ii) and the information given on page 2 to determine the relative formula mass, $M_r$, of the organic acid, HA.
$M_r$ of HA = ..............................
(iv) The organic acid was known to have one of the following structural formulae.
CH₃COOH CH₂ClCOOH CHCl₂COOH CCl₃COOH
Use your answer to (iii) and the Periodic Table on page 16 to identify the anion, A⁻.
anion, A⁻ = ..............................
(d) A student carried out the same procedure accurately but was supplied with a solution of less concentrated sodium hydroxide by mistake.
(i) What effect would this have on the calculated value of the relative formula mass, $M_r$? Explain your answer.
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(ii) Explain how this would affect the identification of the acid.
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563
589
598
You are to determine the enthalpy change of neutralisation for a different acid from that used in Question 1. The acid is represented by HB where \( B^- \) represents the anion.
\[ \text{HB(aq) + NaOH(aq) \rightarrow NaB(aq) + H_2O(l)} \]
FB 3 is \( 2.00\, \text{mol dm}^{-3} \) acid, HB.
FB 4 is \( 72.00\, \text{g dm}^{-3} \) sodium hydroxide, NaOH.
(a) Method
Read through the method before starting your practical work and prepare a table below for recording your results.
Experiment 1
• Place the plastic cup in the \( 250\, \text{cm}^3 \) beaker.
• Pour \( 25\, \text{cm}^3 \) of FB 3 into the larger measuring cylinder.
• Measure and record the temperature of FB 3.
• Rinse and dry the thermometer.
• Use the smaller measuring cylinder to transfer \( 25\, \text{cm}^3 \) of FB 4 into the plastic cup.
• Measure and record the temperature of FB 4.
• Add the \( 25\, \text{cm}^3 \) of FB 3 to FB 4 in the plastic cup and stir the mixture. Measure and record the highest temperature reached.
• Calculate and record the average initial temperature of the solutions.
• Calculate and record the temperature rise.
• Empty the plastic cup, rinse it with water and shake it to remove excess water.
Experiment 2
• Repeat the method given for Experiment 1 using \( 50\, \text{cm}^3 \) of each solution.
• Use the larger measuring cylinder for FB 3 and the smaller measuring cylinder for FB 4.
Results
[Table_1]
(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Show by calculation that in Experiment 1, the number of moles of acid was in excess of the number of moles of sodium hydroxide.
(ii) Calculate the heat energy evolved in Experiment 1.
[Assume that \( 4.2\, \text{J} \) of heat energy changes the temperature of \( 1.0\, \text{cm}^3 \) of solution by \( 1.0^\circ \text{C} \).]
heat energy evolved = \( \text{......................} \) J
(iii) Calculate the enthalpy change, in \( \text{kJ mol}^{-1} \), for Experiment 1.
enthalpy change = \( \text{......} \ \text{..............................} \ \text{kJ mol}^{-1} \)
(sign)\ (value)
(iv) Calculate the number of moles of sodium hydroxide neutralised in Experiment 2.
moles of NaOH = \( \text{..........................} \) mol
(v) Calculate the enthalpy change, in \( \text{kJ mol}^{-1} \), for Experiment 2.
enthalpy change = \( \text{......} \ \text{..............................} \ \text{kJ mol}^{-1} \)
(sign)\ (value)
(c) (i) The accuracy of the larger measuring cylinder is \( \pm 0.5\, \text{cm}^3 \).
The accuracy of the smaller measuring cylinder is \( \pm 0.25\, \text{cm}^3 \).
Calculate the maximum percentage error in the measurement of the volume of FB 3 used in Experiment 2 and the measurement of the volume of FB 4 used in Experiment 2.
Show your working.
maximum \( \% \) error in volume of FB 3 \( \text{........................} \)
maximum \( \% \) error in volume of FB 4 \( \text{........................} \)
(ii) Suggest a change to the method used in (a) that would improve the accuracy of your results.
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589
521
552
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) \( \text{FB 5 and FB 6 are solutions of acids of equal concentration in mol dm}^{-3} \). One solution is a weak acid and the other is a strong acid.
(i) Devise and carry out a chemical test to find out which of FB 5 and FB 6 is the weak acid. Record your test, observations and conclusion in the space below.
(ii) Another acid, FB 7, is a dilute solution of one of hydrochloric, nitric or sulfuric acids.
Carry out the tests in the order given in the table below until you are able to identify FB 7. Record your observations. If any test is unnecessary write 'not needed'.
| test | observations |
|---|---|
| To a 1 cm depth of FB 7 in a test-tube add aqueous silver nitrate. | |
| To a 0.5 cm depth of FB 7 in a boiling tube add a 1 cm depth of aqueous sodium hydroxide and a small piece of aluminium foil and warm. | |
| To a 1 cm depth of FB 7 in a test-tube add aqueous barium chloride or aqueous barium nitrate. |
(b) FB 8 contains a cation listed in the Qualitative Analysis Notes. FB 9 is a solution of an organic salt. Carry out the following tests and record your observations.
| test | observations |
|---|---|
| (i) To a 1 cm depth of FB 8 in a test-tube add aqueous sodium carbonate. | |
| (ii) To a 1 cm depth of FB 8 in a test-tube add a 1 cm depth of aqueous potassium iodide, then add aqueous sodium thiosulfate until in excess. | |
| (iii) To a 2 cm depth of FB 8 in a test-tube add a 1 cm depth of concentrated hydrochloric acid (CARE). Keep this solution for test (iv). | |
| (iv) To a 2 cm depth of distilled water in a boiling tube add all the contents of the test-tube from test (iii). Keep this solution for tests (v) and (vi). Pour a 1 cm depth of the contents of the boiling tube into three separate test-tubes for use in tests (v) and (vi). One tube is to be used for comparing colours in your observations. | |
| (v) To one of the test-tubes add aqueous ammonia until in excess. | |
| (vi) To a second test-tube add FB 9 until in excess. |
cation .................................
(viii) Write an ionic equation for a precipitation reaction you observed during your experiments with this cation. Include state symbols.
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569
578
509
