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Edexcel Jun 2014 (IAL) Paper 4 Q22
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![Answer ALL the questions. Write your answers in the spaces provided. SECTION C 22 Lactic acid, 2-hydroxypropanoic acid, is found in sour milk. The structure of lactic acid, CH3CH(OH)COOH, is shown below. OH H C C C OH (a) (i) State what is meant by a Brnsted-Lowry acid. (1)(ii) Write the equation for the reaction of lactic acid with water to form an acidic solution. State symbols are not required. (iii) Write the expression for the acid dissociation constant, Ka, for lactic acid. (1) (1) *P42992A02328* 23 Turn over<br />
(iv) Calculate the pH of a solution of lactic acid of concentration 0.15 mol dm3 at 298 K. The pKa of lactic acid is 3.86 at 298 K. State clearly any assumptions you have made. Calculation: (5) Assumptions:24 *P42992A02428*<br />
(b) 25 cm3 of a 0.15 mol dm3 solution of lactic acid was titrated with 50 cm3 of sodium hydroxide solution of the same concentration. Part of the titration curve is shown below. pH 141210864200 10 15 20 25 30 35 40 45 50 Volume of NaOH(aq) / cm3 (i) Calculate the pH of 0.15 mol dm3 sodium hydroxide solution. Give your answer to one decimal place. [Kw = 1.0 (cid:117) 1014 mol2 dm6 at 298 K] (ii) Complete the titration curve on the diagram above to show the change in pH until 50.0 cm3 of the sodium hydroxide solution has been added to the solution of lactic acid. (2) (1) *P42992A02528* 25 Turn over<br />
(iii) Use the information given in your Data Booklet to select a suitable indicator for this titration, giving the colour change you would expect to see. Justify your selection. (3)(iv) The equation for the neutralization in the titration is CH3CH(OH)COOH(aq) + NaOH(aq) (cid:314) CH3CH(OH)COONa(aq) + H2O(l) Use the titration curve to suggest the pH of a 0.075 mol dm3 solution of sodium lactate. Justify your answer. (2)26 *P42992A02628*<br />
*(v) Explain, using ionic equations, how a solution containing lactic acid and sodium lactate can act as a buffer solution. (4)(c) Ethanoic acid, CH3COOH, has a pKa of 4.8 at 298 K. Complete the equation to show the conjugate acid-base pairs that could be produced when pure samples of lactic acid and ethanoic acid are mixed. (1) CH3CH(OH)COOH + CH3COOH (cid:314)+(Total for Question 22 = 21 marks) TOTAL FOR SECTION C = 21 MARKS TOTAL FOR PAPER = 90 MARKS *P42992A02728* 27<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJune_2014_IAL_QP_-_Unit_4_Edexcel_Chemistry_A-level_22-0.jpg)
![Section C Acceptable Answers Question Number 22(a)(i) Proton donor/donates protons OR H+ ion donor/donates H+ ions Ignore just releases H+ ions or protons. Acceptable Answers Question Number 22(a)(ii) CH3CH(OH)COOH + H2O+ H3O+ CH3CH(OH)COO + H3O+ OR CH3CH(OH)CO2H + H2O CH3CH(OH)CO2 ALLOWfor ALLOW CH3CH(OH)COOH ALLOW CH3CH(OH)CO2H Ignore state symbols even if incorrect Ignore missing bracket around OH CH3CH(OH)COO + H+ CH3CH(OH)CO2+ H+ Reject Mark Reject Mark HA and A- once only in (a)(ii) and (a)(iii) Penalise missing H once only in (a)(ii) and (a)(iii) Question Number 22(a) (iii) Acceptable Answers Reject Mark ][ H3O+] (Ka =) [CH3CH(OH)COO][ H3O+] [CH3CH(OH)COOH] OR [CH3CH(OH)CO2 [CH3CH(OH)CO2H] OR [CH3CH(OH)COO][ H+] [CH3CH(OH)COOH] OR [CH3CH(OH)CO2 [CH3CH(OH)CO2H] Note Allow any of these for the mark, even if a different equation using H+/H3O+ has been given in (a)(ii) ][ H+] H2O in expression Lack of square brackets HA and A- once only in (a)(ii) and (a)(iii) Penalise missing H once only in (a)(ii) and (a)(iii) WCH0401 1406<br />
Question Number 22(a) (iv) Acceptable Answers Reject Mark Comment Allow [H3O+], [HA], [A-] as alternative formula throughout Calculation Ignore sf, except 1 sf, throughout First check the final answer If pH = 2.34, award the first 3 marks If pH2.34, award marks as follows ROUTE 1 Ka = 10-3.86 = 1.3804 x 10-4(mol dm-3) (1) [H+] = Ka[CH3CH(OH)COOH] = 1.38 x 10-4 x 0.15 = 4.5504 x 10-3 (mol dm-3) (1) TE on their value for Ka pH = -log10[H+] = -log10 4.55 x 10-3 = 2.3420/2.34 (1) TE on their value for [H+] provided pH>1 and < 7 (pH = 0.12 if use 3.86 for Ka, scores 2) ROUTE 2 [H+] = Ka[CH3CH(OH)COOH] pH =pKalog[CH3CH(OH)COOH] (1) =3.86log 0.15 (1) = 2.34 (1) TE on not halving (4.68 is worth 1 mark) Assumption 1 [H+] = [CH3CH(OH)COO-] OR no H+ from the (ionization of) water/ionization of water is negligible OR H+ is (only) from the acid (1) Assumption 2 Ionization/dissociation of the (weak) acid is negligible / very small / insignificant OR [CH3CH(OH)COOH]initial = [CH3CH(OH)COOH]equilibrium OR [CH3CH(OH)COOH]equilibrium = 0.15 (mol dm-3) OR [H+]/ [CH3CH(OH)COO-] << [CH3CH(OH)COOH] OR [CH3CH(OH)COOH]/acid concentration remains constant (1) Incorrect units for Ka and/or [H+], max 2 for calculation [H+]initial = [H+]equilibrium/ WCH0401 1406<br />
ROUTE 3 First check the final answer If pH = 2.35, award the first 4 marks If pH2.35, award marks as follows Ka = 10-3.86 = 1.38 x 10-4 (mol dm-3) (1) [H+]2 = Ka([CH3CH(OH)COOH] - [H+] [H+]2 =1.38 x 10-4x (0.15 -[H+] (1) [H+] = 4.48 x 10-3 (mol dm-3) (1) ecf on their value for Ka pH = -log10[H+] = -log10 4.48 x 10-3 = 2.35 (1) TE on their value for [H+] Assumption [H+] = [CH3CH(OH)COO-] OR no H+ from the (ionisation of) water OR H+ is (only) from the acid (1) [H+]initial = [H+]equilibrium WCH0401 1406<br />
Acceptable Answers Question Number 22(b)(i) If answer is 13.2, with or without working, award 2 marks (13/13.17 score 1 mark, answer not to 1 dp) [H+] = 1.0 x 10-14 0.15 = 6.67 x 10-14 (mol dm-3) (1) pH = -log106.67 x 10-14 = 13.176 = 13.2 (1) TE on their [H+], provided pH > 7 and < 14 OR pOH = 0.824 (1) pH = 14-0.824 = 13.176 = 13.2 (1) Reject Mark Answer not given to 1 dp Answer not given to 1 dp Acceptable Answers Question Number 22(b)(ii) Curve continues and finishes at any pH between 12 and 13.2 ALLOW this as standalone if they have no pH in (b)(i) or their pH is10 TE on their pH in (b)(i) if it is10 Reject Mark finishing at >13.2 or 12 curves that dip by more than 1 small square at the end stopping before 45 cm3 WCH0401 1406<br />
Acceptable Answers Question Number 22(b)(iii) Indicator and colour change Reject Mark Universal indicator loses all 3 marks named indicator (1) matching colour change (1) phenol red yellow to red/pink OR thymol blue (base) yellow to blue OR phenolphthalein colourless to red/pink /magenta ALLOW bromothymol blue yellow to blue ALLOW thymolphthalein if they have continued to vertical section to at least 10.6 with colour change colourless to blue, for both marks ALLOW correct colour change for thymolphthalein even if the vertical section does not continue to 10.6 for 1 mark. NO TE for colour change from any other indicator Justification pH range (of indicator) lies (completely) in the vertical jump (on the titration curve) OR Indicator will change colour in the vertical section of the graph OR pH range of indicator and pH range of vertical section of the graph stated as long as they overlap ALLOW pKin (1) is in the vertical jump OR pKin is nearest to the pH at the end/equivalence point ALLOW Indicator will change colour at the end/equivalence point ALLOW (because it is a) titration of a weak acid with a strong base (1) WCH0401 1406<br />
Question Number 22(b)(iv) Acceptable Answers Reject Mark the concentration of sodium lactate is 0.075 mol dm-3 when equal amounts of acid and base have been mixed OR (only sodium lactate is present and it is the) pH at the equivalence/end point/ halfway up the vertical section of the curve ALLOW explanation or an equation to show that lactate ions react with water to produce an alkaline solution (1) any number or range within 7.59.5 (1) pH of buffer solution 7/7.0 WCH0401 1406<br />
Question Number 22(b)* (v) Acceptable Answers Reject Mark Reservoir of H+ ionsin equation Comment Allow [HA], [A-] as alternative formulae throughout First markstatement or equations showing the buffer solution buffer solution contains a large amount/reservoir /excess of a weak acid and its conjugate base/salt OR a large amount/reservoir /excess of lactic acid and lactate ions/formulae for lactic acid and lactate ions OR CH3CH(OH)COOHCH3CH(OH)COO + H+ / CH3CH(OH)COOH + H2OCH3CH(OH)COO + H3O+ and CH3CH(OH)COO()Na(+)CH3CH(OH)COO + Na+ equation Ignore definitions of a buffer solution (1) Second markidentifying which species react with the added acid and alkali CH3CH(OH)COOH reacts with added alkali/OH ions OR CH3CH(OH)COOH + OHOR OH ions react with H+/H3O+ ions and CH3CH(OH)COO reacts with added acid/H+ ions / H3O+ ions OR CH3CH(OH)COO + H+/ H3O+ OR (pH=pKa + lg[salt] ) [acid] small additions of acid/alkali have little/no effect on lg[salt] so pH hardly changes/no change [acid] ALLOW Ratio [acid]:[salt]/[salt]:[acid] only changes a little so pH hardly changes/no change ALLOW HA and A in formulae/equations (1) Comment This mark may be given from the equationsin WCH0401 1406<br />
Third markionic equation for OH- ALLOW equations with reversible arrows CH3CH(OH)COOH + OH-CH3CH(OH)COO + H2O ALLOW H+ + OHH2O / H3O++ OH2H2O (1) Fourth markionic equation for H+ CH3CH(OH)COO + H+CH3CH(OH)COOH OR CH3CH(OH)COO + H3O+CH3CH(OH)COOH + H2O (1) Note Only penalise non ionic equations e.g. using NaOH, HCl once only. Comment Two completely correct ionic equations scores marks 2,3 and 4 Question Number 22(c) Acceptable Answers CH3CH(OH)COO + CH3COOH2 both needed for the mark ALLOW [CH3CH(OH)COO] + [CH3COOH2]+ ALLOW [CH3COOHH]+ / CH3COOHH+ for the ethanoic acid ion CH3CH(OH)CO2for the lactic acid ion Reject Mark (Total for Question 22 = 21 marks) TOTAL FOR PAPER = 90 marks WCH0401 1406<br />](https://learnah.org/wp-content/uploads/2018/08/June_2014_IAL_MS_-_Unit_4_Edexcel_Chemistry_A-level_22-0.jpg)
