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Edexcel Jun 2012 Paper 4 Q14
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![Reject Mark Numerator as [H+]2 Expressions in terms of HA alone Round/curved brackets () Any other carboxylic acid Reject Mark 4.77 or 4.8 from using pH = -log Ka loses both marks Reject Mark Section B Question Number 14 (a)(i) Acceptable Answers ] [H+]/[CH3CO2H] ] [H3O+]/[CH3CO2H] Ka = [CH3CO2 OR Ka = [CH3CO2 OR Use of [CH3COO] instead of [CH3CO2 and [CH3COOH] instead of [CH3CO2H] IGNORE state symbols even if wrong ] Question Number 14(a)(ii) 1.7 x 10-5 = [H+]2 / 0.5 Acceptable Answers [H+] = 1.7 x 10-5 x 0.5 / 2.915(476) x 103 (1) pH = (-log[H+]) = 2.53529 OR = 2.54 OR = 2.5 (1) ALLOW TE for second mark from any hydrogen ion concentration as long as pH less than 7 Correct answer alone scores (2) ALLOW pH = 2.53 if [H+] is rounded to 2.92 x 103 IGNORE sf except 1 Question Number 14(a)(iii) 20 (cm3) IGNORE units Acceptable Answers OR 0.02 dm3<br />
Acceptable Answers Question Number 14(a)(iv) Moles of excess NaOH = 10/1000 x 0.50 = 5 x 10-3 (1) So [NaOH/OH-] = 5 x 10-3 x 1000/50 = 0.10 mol dm-3 (1) EITHER Reject Mark Kw route: [H+] x 0.1 = 1 x 10-14 (1) So pH = - log 1 x 10-14 / 0.1 = 13 (1) OR pOH route: pOH = 1 (1) So pH = (141) = 13 (1) ALLOW TE throughout Correct final answer scores (4)<br />
Question Number 14(a)(v) Starting at pH 2-3 Acceptable Answers Reject Mark AND finishing at pH between 12 and 13.7 inclusive (1) Vertical section at 20 cm3 (1) S-shaped curve, with gradual rise and vertical section within the pH range 5.5 and 11.5 and of 3 to 5 units in length (1) These are stand alone marks<br />
Question Number 14(b)(i) EITHER Acceptable Answers Reject Mark [base] = Ka [acid]/[H+] Or [H+] = (10-pH4.70) = 1.995 x 10-5 (1) [base] = 1.7 x 10-5 x 1/(1.995 x 10-5) = 0.852 (1) moles base = 0.852 x 0.5 = 0.426 (mol) (1) mass base = 0.426 x 82 = 34.9 g (1) IGNORE sf except 1 Correct answer, with or without working (4) OR pH = pKalog[acid]/[base] 4.70 = 4.8log [1/[base]] Log[1/[base]] = 0.1 (1) [base] = 0.794(328) (mol dm-3) (1) So in 500 cm3 Moles = 0.794 x 0.5 = 0.397 mol (1) Mass = 0.397 x 82 = 32.554/32.6 g (1) (ALLOW using pKa = 4.77)<br />
Reject Mark Acceptable Answers First mark Buffer has large amount/ excess/ reservoir of CH3COOH (and CH3COO) (1) Second mark OH ions added react with CH3COOH OR CH3COOH + OHCH3COO + H2O OR OH + H+H2O and CH3COOHCH3COO + H+ OR Equations described in words (1) Third mark Ratio / values of [CH3COOH] to [CH3COO] remains (almost) unchanged (1) IGNORE concentration of hydrogen ions remains constant ALLOW answers in terms of HA and A Question Number 14(b)(ii)<br />
Reject Mark Reject Mark C9H18O KMnO4 (0) for last 2 marks HCl (0) for 3rd mark Reject Mark Acceptable Answers 2,6-dimethylhept-5-enal (2) Either part scores (1) e.g. 2,6-dimethyl (1) hept-5-enal (1) IGNORE missing/misplaced/misused hyphens or commas ALLOW ene for en ALLOW methy or methly for methyl Question Number 15(b)(i) CH3C(CH3)=CHCH2CH2CH(CH3)CH2OH (1) Acceptable Answers OR CH3C(CH3)CHCH2CH2CH(CH3)CH2OH OR CH3C(CH3)=CHCH2CH2C (CH3) HCH2OH ALLOW displayed or skeletal formulae K2Cr2O7/Na2Cr2O7/name (oxidation state must be correct if given (VI)) (1) This is a stand alone mark H2SO4/name (ignore any references to concentration) (1) ALLOW H+ and Cr2O7 Acidified dichromate (1) 2- (2) Acceptable Answers Question Number 15(b)(ii) (Steam) distil off melonal (as it forms) Allow add a limited amount of oxidizing agent/excess alcohol/excess X (1) To prevent further oxidation/To prevent carboxylic acid forming (1) Stand alone marks Question Number 15(a)<br />](https://learnah.org/wp-content/uploads/2018/08/June_2012_MS_-_Unit_4_Edexcel_Chemistry_A-level_14-0.jpg)
