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Edexcel Jun 2011 Paper 4 Q18
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![Question Number 18 (a) Question Number 18 (b)(i) Question Number 18 (b)(ii) Question Number 18 (b)(iii) Mark Mark Mark Acceptable Answers Reject Ka = (10-10.64) = 2.3 x 10-11 / 2.2909 x 10-11 (mol dm-3) Ignore sf except 1 Acceptable Answers Reject Ka = [H+]2 [HCOOH] without also giving full expression Reject pH =2 pH =2.1 - and HCO2H Ka = [HCOO-][H+] [HCOOH] OR written as HCO2 OR with H3O+ instead of H+ Allow Ka = [A-][H+] [HA] if formula of HA and A- given as HCOOH and HCOO- Acceptable Answers 1.6 x 10-4 = [H+]2 0.50 (1) [H+] = 1.6 x 10-4x0.5 (1) (= 8x10-5 = 8.94 x 10-3 ) pH = (2.048455) = 2.05 / 2.0 (1) Correct answer with no working (3) TE for third mark if [H+] calculated incorrectly No TE from incorrect Ka expression Ignore sf except 1 Acceptable Answers Reject All H+ comes from acid / none from water / [H+] = [HCOO-] OR [H+] = [A-] OR Dissociation of acid is negligible / very small OR [HA]initial =[HA]equilibrium Ka is measured at 298K Just dissociation of acid is partial Mark 6CH04/01 1106<br />
Question Number 18 (c)(i) Question Number 18 (c)(ii) Acceptable Answers HCOOH CH3COOH2 both correct (1) Acceptable Answers (HIO + CH3COOH ) H2IO+ + CH3 COO- / (HIO + CH3COOH ) HIOH+ + CH3 COO- Ignore position of positive charges Reject Reject Mark Mark 6CH04/01 1106<br />
Question Number 18 (d) Reject Mark Acceptable Answers (pH = 4.9) so [H+] = (1.2589254 x 10-5 ) =1.259 x 10-5 (1) ( Ka = [HCOO-] [H+] [HCOOH] = 1.6 x 10-4 1.259 x 10-5 ) = 12.7 (:1) / 13(:1) (HCOO- per HCOOH or base:acid) (12.709252 from unrounded [H+] 12.708499 from [H+] rounded to 1.259 x10-5 12.3 from [H+] rounded to 1.3 x10-5) TE from error in [H+] Allow 800:63 (1) Correct answer scores 2 Accept (0.0786828) = 0.079 HCOOH per HCOO- for acid:base ratio (0.0786874) = 0.079 from rounded pH OR pKa = -log Ka = 3.79 3.79 = 4.9log [base] (1) [acid] log [base] = 1.11 [acid] [base] = (12.882496) = 12.9 (:1) (1) [acid] Correct answer scores 2 Accept 0.0776/ 0.078 HCOOH per HCOO- for acid:base ratio (0.0776247) TE from error in pKa Ignore sf except 1 TOTAL FOR SECTION B = 50 MARKS 6CH04/01 1106<br />](https://learnah.org/wp-content/uploads/2018/08/June_2011_MS_-_Unit_4_Edexcel_Chemistry_A-level_18-0.jpg)
