Skip to content
AQA Specimen Paper 3 Q2
Answers available below
Show answer
Question
1
2
3
4
5
![Ethanol and ethanoic acid react reversibly to form ethyl ethanoate and water according to the equation: CH3COOH + CH3CH2OH CH3COOCH2CH3 + H2O A mixture of 8.00102 mol of ethanoic acid and 1.20101 mol of ethanol is allowed to reach equilibrium at 20 C.The equilibrium mixture is placed in a graduated flask and the volume made up to 250 cm3 with distilled water.A 10.0 cm3 sample of this equilibrium mixture is titrated with sodium hydroxideThe ethanoic acid in this sample reacts with 3.20 cm3 of 2.00101 mol dm3 added from a burette. sodium hydroxide solution. . 1 0 2 Calculate the value for Kc for the reaction of ethanoic acid and ethanol at 20 C. Give your answer to the appropriate number of significant figures. [6 marks] Kc = Question 2 continues on the next page Barcode Typesetter code Turn over <br />
A student obtained the titration results given in Table 2. Table 2 Rough Final burette reading / cm3 Initial burette reading / cm3 Titre / cm3 4.60 0.10 8.65 4.65 12.85 16.80 8.65 12.85 . 2 Complete Table 2. Calculate the mean titre and justify your choice of titres. . 3 0 2 0 2 [1 mark] [2 marks] Calculation Justification Mean titre = cm3 . 4 0 2 The pH ranges of three indicators are shown in Table 3. Table 3 Indicator Bromocresol green Bromothymol blue Thymol blue pH range 3.85.4 6.07.6 8.09.6 Select from Table 3 a suitable indicator for the titration of ethanoic acid with sodium hydroxide. [1 mark] Barcode Typesetter code<br />
0 2 . 5 The uncertainty in the mean titre for this experiment is 0.15 cm3. Calculate the percentage uncertainty in this mean titre. [1 mark] Percentage uncertainty = . 6 0 2 Suggest how, using the same mass of ethanoic acid, the experiment could be improved to reduce the percentage uncertainty. [2 marks] Turn over for the next question Barcode Typesetter code Turn over <br />](https://learnah.org/wp-content/uploads/2018/08/Specimen_QP_-_Paper_3_AQA_Chemistry_A-Level_2-0.jpg)
![MARK SCHEMEA-level ChemistrySpecimen paper 3 Marking guidance Mark AO Comments Stage 1: Moles of acid at equilibrium Moles of sodium hydroxide in each titration = (3.202.00101) / 1000 = 6.40104 Sample = 10 cm3 so moles of acid in 250 cm3 of equilibrium mixture = 256.40104 = 1.60102 Stage 2: Moles of ester and water formed Moles of acid reacted = 8.001021.60102 = 6.40102 = moles ester and water formed Stage 3: Moles of ethanol at equilibrium Moles of ethanol remaining = 1.201016.40102 = 5.60102 Stage 4: Calculation of equilibrium constant Kc = [CH3COOCH2CH3] [H2O] / [CH3COOH] [CH3CH2OH] 1 = (6.40102)2 / (1.60102)(5.60102) = 4.5714 = 4.57 1 1 1 AO2h Extended response AO2h M2 can only be scored if = answer to M125 M3 is 8.00102M2 M4 is 1.20101M3 M6 is M32 / M2M4 Answer must be given to 3 significant figures 4 of 10 AO2h AO2h AO1b AO2h Question 02.1<br />
02.2 Rough Final burette reading / cm3 4.60 8.65 12.85 16.80 Initial burette reading / cm3 0.10 4.65 8.65 12.85 Titre / cm3 4.50 4.00 4.20 3.95 Mean = 4.00 + 3.95 / 2 = 3.98 (cm3) Titres 1 and 3 are concordant Thymol blue Percentage uncertainty: 0.15/3.98100 = 3.77% Use a lower concentration of NaOH So that a larger titre is required (reduces percentage uncertainty in titre) 02.3 02.4 02.5 02.6 MARK SCHEMEA-level ChemistrySpecimen paper 3 1 1 1 1 1 AO1b AO3 1a AO3 1a AO1b Allow 3.975 (cm3) Allow titre 2 is not concordant AO2h Allow consequential marking on mean titre from 2.3 AO3 2b AO3 2b 5 of 10<br />](https://learnah.org/wp-content/uploads/2018/08/Specimen_MS_-_Paper_3_AQA_Chemistry_A-Level_2-0.jpg)
