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HA is an organic acid. Solution FA 1 was prepared by dissolving 13.1 g of solid HA in each dm³ of solution. You are to determine the relative molecular mass, $M_r$, of HA by titration with aqueous sodium hydroxide. The equation for the reaction between HA and sodium hydroxide is given below.
$$ \text{HA(aq) + NaOH(aq) → NaA(aq) + H}_2\text{O(l)} $$
FA 1 is a solution containing 13.1 g dm−3 of organic acid, HA.
FA 2 is 0.100 mol dm−3 sodium hydroxide, NaOH.
bromothymol blue indicator
(a) Method
• Fill the burette with FA 1.
• Use the pipette to transfer 25.0 cm³ of FA 2 into a conical flask.
• Add a few drops of bromothymol blue indicator. This indicator is blue in alkali and yellow in acid 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 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.
[Image of table]
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(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value.
25.0 cm³ of FA 2 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 sodium hydroxide present in the volume of FA 2 pipetted into the conical flask.
moles of NaOH = ......................... mol
(ii) Use your answer to (c)(i) and the equation on page 2 to determine the number of moles of organic acid, HA, used to neutralise 25.0 cm³ of FA 2.
moles of HA = ............................. mol
(iii) Use your answers to (b) and (c)(ii) to calculate the number of moles of HA in 1 dm³ of FA 1.
moles of HA in 1 dm³ of FA 1 = ........................... mol
(iv) Calculate the relative molecular mass, $M_r$, of the organic acid, HA.
$M_r$ of HA = ........................... [4]
(d) A student carrying out this method correctly with the same concentration of reactants obtained a titre of 28.30 cm³. Would this give a larger or smaller value of $M_r$ than yours? Explain your answer.
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You are to determine the enthalpy change for the neutralisation reaction given below.
$HA(aq) + NaOH(aq) \rightarrow NaA(aq) + H_2O(l)$
You will be using solutions of different concentrations from those in Question 1.
FA 3 is $1.80 \text{ mol dm}^{-3}$ HA.
FA 4 is aqueous sodium hydroxide, NaOH.
(a) Method
Read through the instructions carefully and prepare a table below for your results before starting any practical work.
• Support the plastic cup in the $250 \text{ cm}^3$ beaker.
• Rinse and fill the burette with FA 3.
• Use the measuring cylinder to transfer $25\text{ cm}^3$ of FA 4 into the plastic cup.
• Place the thermometer in the plastic cup and record the temperature of the solution. Tilt the cup if necessary to ensure the thermometer bulb is fully immersed.
• Run $5.00\text{ cm}^3$ of FA 3 into the cup. Stir, and record the new temperature of the solution and the volume of FA 3 added.
• Run a second $5.00\text{ cm}^3$ of FA 3 into the cup. Stir and record the new temperature and the total volume of FA 3 added.
• Continue adding FA 3 in $5.00\text{ cm}^3$ portions. Stir and record each new temperature and total volume of FA 3 until a total of $45.00\text{ cm}^3$ has been added.
Results
(b) Plot a graph of temperature (y-axis) against total volume of FA 3 added (x-axis) on the grid opposite. The temperature axis should allow you to include a point at least $2^\circ\text{C}$ greater than the maximum temperature recorded.
Draw the best fit smooth curve or straight line through the two sets of points, one for the increase in temperature of the mixture and the other for the cooling of the solution once the reaction is complete. Extrapolate the two lines and determine the maximum increase in temperature and the corresponding volume of FA 3 added for this increase in temperature.
maximum temperature increase = ................. $^\circ\text{C}$
volume of FA 3 = ................. $\text{cm}^3$
(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 HA present in the volume of FA 3 recorded in (b).
moles of HA = .............................. mol
(ii) Using your answers to (b), calculate the heat energy produced when FA 3 neutralised $25\text{ cm}^3$ of sodium hydroxide.
(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 produced = .............................. $\text{J}$
(iii) Calculate the enthalpy change of neutralisation, in $\text{kJ mol}^{-1}$, for the reaction below.
$HA(aq) + NaOH(aq) \rightarrow NaA(aq) + H_2O(l)$
enthalpy change = ...... ................ $\text{kJ mol}^{-1}$
(sign) (value)
(d) The maximum error in a single thermometer reading is $\pm0.5^\circ\text{C}$.
Calculate the maximum percentage error in the increase in temperature recorded in (b).
maximum percentage error = .............................. %
(e) When carrying out thermochemistry experiments in an A Level laboratory, the plastic cup is usually placed in a glass beaker. Give a reason for the use of the glass beaker.
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(f) Apart from using a thermometer calibrated to a greater level of precision, suggest one improvement that could be made to the method carried out in (a).
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(a) FA 5 is the salt of another organic acid.
FA 6 is a salt containing two cations and one anion from those listed in the Qualitative Analysis Notes on pages 10 and 11.
Carry out the following tests and record your observations in the table below.
test | observations
(i) To a 1 cm depth of dilute sulfuric acid in a test-tube, add five drops of potassium manganate(VII) then a small spatula measure of FA 5. Shake the tube then place it in the hot water bath.
(ii) Place a spatula measure of FA 6 in a hard-glass test-tube. Heat gently and test any gas evolved with litmus paper.
(iii) To a 4 cm depth of water in a boiling tube, add and dissolve a spatula measure of FA 6. Use portions of this solution, aqueous FA 6, for tests (iv)–(vi).
(iv) To a 1 cm depth of aqueous FA 6 in a test-tube, add aqueous ammonia.
(v) To a 1 cm depth of aqueous FA 6 in a test-tube, add a 1 cm depth of aqueous barium chloride followed by dilute hydrochloric acid or a 1 cm depth of aqueous barium nitrate followed by dilute nitric acid.
(vi) To a 1 cm depth of aqueous FA 6 in a test-tube, add a small spatula measure of FA 5 and shake the mixture. Keep this mixture for the test in part (b).
(vii) Identify as many of the ions in FA 6 as possible from your observations.
FA 6 contains .............................................................
(b) A student suggests that one of the cations in FA 6 was oxidised in test (a)(vi).
(i) Name a reagent you would use to find out whether the student was correct.
reagent .............................................................
(ii) Use your reagent to carry out a test on the mixture from (a)(vi). Record your observations below. State and explain whether the student was correct.
observations .............................................................
conclusion .............................................................
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