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Edexcel Jan 2010 Paper 4 Q21
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![Section B Question Number 21 (a)(i) Question Number 21 (a)(ii) Mark Acceptable Answers Reject Just concentration of hydrogen ions { } curly brackets -log H+ (pH =) -log [H+] OR (pH =) -log [H3O+] OR Accept Definition in words (For example: It is minus / negative log(arithm) of the hydrogen ion concentration) Base 10 does not have to be there, but reject ln Acceptable Answers Reject Mark (pH = -log 0.0100) = 2(.00) If any units given<br />
Mark Acceptable Answers Reject [H3O+] = Ka[CH3COOH] [CH3COO] OR [H3O+]2 = Ka[CH3COOH] (1) ALLOW [HA] for [CH3COOH] and [A-] for [CH3COO-] in rearranged expression Accept [H+] for [H3O+] [H3O+] =1.75 x 107 OR[H3O+] = 4.18(3) x 104 (mol dm-3) (1) pH = 3.38 / 3.4 (1) ignore sf except one sf Third mark TE from [H+] only if pH less than 7 N.B. CORRECT ANSWER, WITH OR WITHOUT WORKING, SCORES (3) Assumption assumes that degree of ionisation of the acid is very small/negligible OR [CH3COOH ]eqm =[CH3COOH ]initial OR [H+] = [CH3COO] OR all of the hydrogen ions come from the acid / ignore hydrogen ions from the water (1) IGNORE any references to temperature 3.37 / 3 /3.39 / a correct pH value with units just weak acid / just partially dissociates / acid does not dissociate / [CH3COOH] constant [H+] = [OH] / [H+] = [salt] Question Number 21 (b)(i)<br />
Mark Acceptable Answers Reject Reject just a reference to a 0.5 increase in pH for CH3COOH(aq) compared with a 1.0 increase in pH for HCl(aq) First mark: (Dilution/addition of water) shifts the equilibrium CH3COOHCH3COO + H+ / CH3COOH + H2OCH3COO + H3O + to the right OR the above stated in words such as: degree of dissociation increases/ proportion of dissociation increases/ more dissociation (as the ethanoic acid is diluted) (1) Second mark: so the [H+] is greater than expected/ so the decrease in [H+] is less than expected / so that the decrease in [H+] is less than that for hydrochloric acid (1) Each mark is a stand alone mark. ALTERNATIVE ROUTE: First mark: [H+] = Ka [HA] OR (Ka [HA]) OR pH = pKa - log[HA] (1) Second mark: use of mathematical expression given (e.g.[H+] affected by factor of 1/10 on dilution OR substitution of numerical values into the equation) (1) IGNORE: any comments or calculations relating to HCl(aq) Question Number 21 (b)(ii)<br />
Question Number 21 (c)(i) Mark Acceptable Answers Reject resists small change(s) in pH OR pH does not change These marks are stand alone. Maintains an almost constant pH / resists change(s) in pH (1) for small addition of H+ or OH- ions (N.B. both ions needed) / for small additions of acid or alkali / for small additions of acid or base (1) IGNORE any references to named buffer mixtures Question Number 21 (c)(ii) Acceptable Answers citric acid Reject Mark<br />
Acceptable Answers Reject First mark: (buffer contains) reservoir of HA and A OR (buffer contains) large concentrations of [HA] and [A-] OR both equations: HAA + H+ and NaA Na+ + A (1) Mark JUST NaANa+ + A and HA H+ + A without correct description Just [H+] remains constant Second mark: (Addition of alkali/base) HA + OHA + H2O OR description/equations to show that H+ reacts with OH (to form H2O) and more acid dissociates (to replace H+) (1) Third mark: (Addition of acid) A + H+HA OR A reacting with H+ in any context described in words (e.g. by reference to weak acid equilibrium) (1) Fourth mark: the ratio of [A][HA] hardly changes / the ratio of [HA] [A] hardly changes OR [A] nor [HA] changes significantly (1) Question Number 21 (c)(iii)<br />](https://learnah.org/wp-content/uploads/2018/08/January_2010_MS_-_Unit_4_Edexcel_Chemistry_A-level_21-0.jpg)
