M1. (a) ph = log[h + ] 1. ph = 2.79 (penalise 1 dp or more than 2dp once in the qu) despite the addition of (small amounts of) acid/base (or dilution)

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Damian Wilkins
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1 M. (a) ph = log[h + ] [H + ]= (or ) ph = 2.79 (penalise dp or more than 2dp once in the qu) (b) (i) Solution which resists change in ph /maintains ph despite the addition of (small amounts of) acid/base (or dilution) (ii) CH 3 COO + H + CH 3 COOH must show an equation full or ionic in which ethanoate ions are converted to ethanoic acid (c) (i) if rearrangement incorrect, no further marks (= ) ph = 4.58 Page of 34

2 (ii) Ml moles H + added = = 0.0 M2 moles ethanoic acid after addition = = 0.6 M3 moles ethanoate ions after addition = = 0.09 M4 (= ) M5 ph = 4.5 The essential part of this calculation is addition/subtraction of 0.0 moles to gain marks M2 and M3. If both of these are missing, only mark M is available. Thereafter treat each mark independently, except if the expression in M4 is wrong, in which case both M4 and M5 are lost. alternative scheme for part (c)(i) [5] pk a = 4.76 alternative for penultimate mark of part (c)(ii) M2. (a) moles HA = 0.50 = () vol NaOH = = dm 3 () or 8.75 cm 3 2 Page 2 of 34

3 (b) (i) ph = log 0 [H + ] () (ii) Value above 7 but below () (iii) phenol red / thymol blue / phenolphthalein / thymolphthalein i.e. indicator with 7 < pk in < 3 (c) (i) Only slightly dissociated/ionised () NOT not fully dissociated / ionised (ii) K a = () NOT (iii) For weak acid alone: Ka = () [H + ] = = () ph = 2.69 () ph should be given to 2 decimal places penalise answer to d.p. once in question 5 (d) moles OH added = = moles A = moles HA left () or [A ] = [HA] Ka = [H + ] or ph = pk a () ph = 4.56 () 3 [3] M3. (a) (i) K a = () Page 3 of 34

4 (ii) () K a = () (2) [H + ] = = () (3) ph = -log 0 [H + ] () can score independently (4) ph = 2.7 () 2 d.p. essential If forget can score () and (3) for ph = (b) (i) moles acid = () = = x = 25 or = 36.7 (or 37) cm 3 (or 36.6) () NOT 36 NOR 37.0 units must match (ii) Indicator: thymol blue () Explanation: weak acid strong base () equivalent at ph > 7 () or high ph 5 (c) () mol NaOH added = = () If wrong M r : CE lose marks () and (2) then mark on consequentially max 4 (2) mol CH 3 COOH left = = 0.70 () (3) mol CH 3 COO formed = () (4) [H + ] = Ka OR ph = pka + etc () If expression wrong no marks for 4 / 5 / 6 can score () to (4) in (5) (5) [H + ] = OR ph = log () Page 4 of 34

5 (6) ph = 4.23 () Correct answer gets ()()()()()() Mark (5) is for use of correct values of (acid moles) and (salt moles) if one wrong allow ph conseq if both wrong, no further marks e.g. if candidate forgets substitution in (2) he loses (2) and (5) but can score () (3) (4) (6) = max 4 for ph = 4.2 if upside down; answer 5.29 scores 3 for () (2) (3) 6 [6] M4. (a) before any KOH added: K a = or () K a = () [H + ] = = () ph = 2.78 () 4 (b) at 8 cm 3 KOH: Moles KOH added = (8 0 3 ) 0.20 = () moles of CH 3 COO formed = () Original moles of CH 3 COOH = ( ) 0.60 = () moles of CH 3 COOH left = ( ) ( ) = () [H + ] = K a () = = () ph = 4.62 () It forget subtraction : max 5 If K a expression not used max 5 if moles of CH 3 COOH wrong but substitution used max 5 7 Page 5 of 34

6 (c) at 40 cm 3 of KOH: Total moles of KOH = ( ) 0.2 = () excess moles of KOH = ( ) - ( ) = () in total volume = = 65 cm 3 () [OH ] = = () [H + ] = OR poh =.7 = () ph = 2.83 () If volume missed : max 4 If moles of acid wrong but method includes subtraction : max 5 If no subtraction : max 4 6 [Max 6] M5. (a) Hydrogen bonding () between H 2 O and NH 3 () 2 (b) (i) NH 3 + H 2 O NH 4+ + OH () (ii) Ammonia is weak base () NOT partially ionised Equilibrium to left or incomplete reaction () 3 (c) A proton donor () (d) Buffer solution: A solution which resists change in ph () when small amounts of acid or base added or on dilution () Reagent: NH 4 Cl () Allow a correct strong acid 3 Page 6 of 34

7 Allow CH 3 CO 2 H, CH 3 CO 2 H 2+, CH 3 C + (OH) 2 (e) (i) K a = [H + ] [A ] / [HA] () = [H + ] [0.25 4] () /.00 [H + ] = / = () ph = log 0 [H + ] = 4.47 () Allow ph conseq to [H + ] if 2 place decimals given (ii) H + + CH 3 COO CH 3 COOH () 5 [4] M6. (a) CH 3 COOH + H 2 O CH 3 COO - + H 3 O + OR CH 3 COOH CH 3 COO - + H + Must show - Allow CH 3 CO 2 H, CH 3 CO 2 Ignore state symbols (b) (CH 3 COOH + HNO 3 CH 3 COOH NO 3 - ) IGNORE (c) (i) ( new [HNO 3 ] = [H + ] = ) M [H + ] = 8.3(3) 0-3 (mol dm -3 ) OR new[hno 3 ] = M2 ph = log M OR 2.08 Must be 2dp Allow correct ph conseq to their [H + ] concentration Page 7 of 34

8 (ii) M mol NaOH (= ) = M2 Subtraction of M from moles of HNO 3 ( or conseq from c(i)) Excess mol H + = M2 allow ecf for subtraction of mol If no subtraction, no further marks M3 [H + ] = = M3 if no use of volume, no further marks (ph=3.5) If incorrect volume used, can score M4 M4 ph = -log M3 OR 2.32 M4 Allow 2.33 Must be 2 dp (d) (i) M K a = Penalise ( ) once here Not [H+][A-] / [HA] If K a expression wrong Allow correct ph conseq to their [H + ] concentration M4 only M2 K a = or with numbers or with HA M3 [H + ] = [ ( )] = Mark for answer M4 ph = 3.33 Must be 2dp Allow correct ph conseq to their [H + ] concentration (ph = 3.83 can score M, M2 and M4) Page 8 of 34

9 (ii) Sodium ethanoate Ignore formula Allow sodium acetate (iii) M [H + ] = Accept or M2 If M incorrect CE=0 Inclusion of in calculation can only score M M3.2(0) Ignore units gives.24 (e) M (Electronegative) chlorine withdraws electrons Allow Cl has negative inductive effect M2 Stabilises/reduces charge on COO- OR weakens O-H bond OR makes O-H more polar Ignore chloroethanoic acid dissociates more readily Mark independently (f) M Strong acids (almost) completely dissociated/ionised OR not an equilibrium OR equilibrium lies far to the right Cannot have K a value for a reaction not in equilibrium scores both marks M2 K a value for strong acids tends to infinity/is very large OR can t divide by zero in K a [20] M7. (a) Concentration of acid: m v = m 2 v 2 hence 25 m = OR moles NaOH = ; m = /25= 0.09; Page 9 of 34

10 (b) (i) K a =[H + ][A ]/[HA] not K a = [H + ] 2 / [HA]; (ii) pk a = logk a ; (iii) [A ] = [HA]; hence K a = [H + ] [A ] / [HA] = [H + ] and logk a = log[h + ]; (c) ratio [A ] : [HA] remains constant; hence as [H + ] = K a [HA] / [A ]; [H + ] remains constant; (d) (i) ph of mol dm- 3 HCl = 0.60 and ph of 0.50 mol dm- 3 HCl = 0.82; ph change = 0.22; (ii) moles HCl = = v = OR v = / =50; water added = = 20 cm 3 ; [2] M8. (a) K a = (All three sets of square brackets needed, penalise missing brackets or missing charge once in the question) (Don t penalise extra [H + ] 2 /[HA]) Page 0 of 34

11 (b) K a = or [H + ] = [A ] [H + ] = = ph = 2.22 (must be to 2dp) (allow 4th mark consequential on their [H + ]) (c) (i) ph (almost) unchanged (Must be correct to score explanation) H + removed by A forming HA or acid reacts with salt or more HA formed (ii) [H + ] = = or [A ] = = = 0.4 (mol dm 3 ) (Allow 0.39 to 0.4 and allow 0.4) (If not used 3.59, to find [H + ] can only score M2 for working) (If 3.59 used but [H + ] is wrong, can score M2 for correct method and conseq M4) If wrong method and wrong expression, can only score M) Page of 34

12 Allow CH 3 CO 2 H etc (ii) Alternative scheme for first three marks of part (c)(ii) ph = pk a log pk a = = 3.84 log [] M9. (a) (i) log[h + ] or log/[h + ] penalise ( ) (ii) [H + ] = 0.56 mark for the answer; allow 2dp or more [H 2 SO 4 ] = ½ 0.56 = 0.28 (b) (i) CH 3 COOH + NaOH CH 3 COONa + H 2 O OR CH 3 COOH + OH CH 3 COO + H 2 O (ii) mol acid = ( ) 0.4 = or [NaOH] = / = 0.45(4) mark for answer if not look back for error OR [NaOH] = / = or 0.46 Page 2 of 34

13 (iii) (iv) cresol purple NaOH reacts with carbon dioxide (in the air) (c) (i) K a = allow molecular formulae or minor slip in formulae penalise ( ) allow H 3 O + not allow HA etc (ii) K a = or with numbers allow HA etc here This can be scored in part (c)(i) but doesn t score there. [H + ] = ( ( ) = ( )) = mark for or either gives 2.57 ph = 2.57 can give three ticks here for (c)(ii) penalise decimal places < 2 > ph mark conseq on their [H + ] so 5.5 gets 2 marks where square root not taken (iii) M mol OH = ( ) 0.0 = If no subtraction or other wrong chemistry the max score is 3 for M, M2 and M4 M2 orig mol HA = ( ) 0.4 = or or M3 mol HA in buffer = orig mol HA mol OH = or If A is wrong, max 3 for M, M2 and M3 or use of ph = pka log [HA]/[A ] M4 mol A in buffer = mol OH = Mark is for insertion of correct numbers in correct expression for [H + ] Page 3 of 34

14 M5 [H + ] = (= or ) M6 ph = 3.79 can give six ticks for 3.79 if [HA]/[A ] upside down lose M5 & M6 If wrong method e.g. [H + ] 2 /[HA] max 3 for M, M2 and M3 Some may calculate concentrations [HA] = and [A ] = and rounding this to gives ph = 3.80 (which is OK) NB Unlike (c)(ii), this ph mark is NOT awarded conseq to their [H + ] unless following AE BEWARE: using wrongly instead of gives ph = 3.75 (this gets 3 for M, M2 & M4) [8] M0. (a) (i) log[h + ] or log /[H + ] penalise missing square brackets here only (ii) 0.8 2dp required, no other answer allowed (iii) M mol H + = if wrong no further mark if allow M but not M2 for 2.82 M2 ph = 2.8 allow more than 2dp but not fewer Page 4 of 34

15 (b) M [H + ] = M2 K a = or or using numbers do not penalise ( ) or one or more missing [ ] M3 [HX] = M4 [HX] = allow conseq on their [H + ] 2 /( ) (AE) if upside down, no further marks after M2 allow (c) M extra/added OH removed by reaction with H + or the acid M2 correct discussion of equn shift i.e. HX H + + X moves to right OR ratio remains almost constant (d) (i) M mol HY = ( ) = OR [Y] =.0236 mark for answer M2 [H + ] = OR = [H + ] OR [H + ] = OR = [H + ] must be numbers not just rearrangement of Ka expression If either HY value or Y value wrong, (apart from AE -) lose M2 and M3 Page 5 of 34

16 M3 [H + ] = mark for answer M4 ph = 4.9 allow more than 2dp but not fewer allow M4 for correct ph calculation using their [H + ] (this applies in (d)(i) only) If Henderson Hasselbalch equation used: M mol HY = ( ) = OR [Y] =.0236 mark for answer M2 pka = 4.87 M3 log = log = If either HY value or Y value wrong, (apart from AE-) lose M3 and M4 M4 ph = 4.87 ( 0.043) = 4.9 allow more than 2dp but not fewer (ii) Can score full marks for correct consequential use of their HY and Y values from d(i) M Mol HY after adding NaOH = = AE in subtraction loses just M If wrong initial mol HY (i.e. not conseq to part d(i)) or no subtraction or subtraction of wrong amount, lose M and M3 M2 Mol Y after adding NaOH = = AE in addition loses just M2 If wrong mol Y (i.e. not conseq to part d(i)) or no addition or addition of wrong amount lose M2 and next mark gained Page 6 of 34

17 M3 [H + ] = (= ) if convert to concentrations [H + ] = (= ) if HY/Y upside down, no further marks M4 ph = 4.93 allow more than 2dp but not fewer NOT allow M4 for correct ph calculation using their [H + ] (this allowance applies in (d)(i) only) If Henderson Hasselbalch equation used: Can score full marks for correct consequential use of their HY and Y values from d(i) M Mol HY after adding NaOH = = AE in subtraction loses just M If wrong initial mol HY (i.e. not conseq to part d(i)) or no subtraction or subtraction of wrong amount lose M and M3 M2 Mol Y after adding NaOH = = AE in addition loses just M2 If wrong mol Y (i.e. not conseq to part d(i)) or no addition or addition of wrong amount lose M2 and next mark gained M3 log = if HY/Y upside down, no further marks M4 ph = 4.87 ( 0.062) = 4.93 allow more than 2dp but not fewer [8] M. (a) (i) B; C; A; Page 7 of 34

18 (ii) cresolphthalein OR thymolphthalein; (b) (i) log[h + ]; (ii) [H + ] = (or.26 or.3) OR OH = 4 ph; OR = 2.0; = 7.9(4) 0 3 ; (if [H + ] is wrong allow for [OH] = K W /[H + ] or as numbers) (c) (i) K a = [H + ] 2 /[CH 3 CH 2 COOH] OR [H+] 2 /[HA] OR [H + ] = [A ] etc; [H + ] = l.35 l or expression without numbers; =.257 l0 3 ph = 2.90; Page 8 of 34

19 (iii) K a = [H + ] OR pk a = ph; ph = 4.87; (penalise dp once) [3] M2. (a) (i) [H + ][OH ] log [H + ] (ii) [H + ] = [OH ] (iii) ( ) 0.5 = (iv) [H + ] = (= ) ph = 0.40 (b) (i) K a = [H + ][CH 3 CH 2 COO - ] [CH 3 CH 2 COOH] = [H + ] [CH 3 CH 2 COOH] [H + ] = ( ) 0.25 (= ) ph = 2.89 Page 9 of 34

20 (c) (i) ( ) 0.25 = (ii) ( ) (.0 0 3) = (iii) mol salt formed = [H + ] = K a [CH 3 CH 2 COOH] [CH 3 CH 2 COO ) = ( ) (= ) ph = 4.5 [6] M3. (a) Proton donor or H + donor Allow donator (b) (i) B B Both need to be correct to score the mark (ii) A A Both need to be correct to score the mark (iii) B A Both need to be correct to score the mark (c) M [H + ] = 0.25 OR M2 mol HCl = ( ) (= ) Mark for Working M3 vol = dm 3 or 37.8 cm 3 allow dm 3 or cm 3 Units and answer tied Lose M3 if total given as ( ) = 62.8 cm 3 Ignore vol added = 2.8 cm 3 after correct answer Page 20 of 34

21 (d) (i) 4.52 Must be 2dp (ii) K a = ignore = but this may score M in (d)(iii) Must have all brackets but allow ( ) Allow HA etc NO mark for 0 pka (iii) M K a = or with numbers Allow [H + ] = (Ka [HA]) for M M2 [H + ] = ( ( ) = ( ) ) = Mark for answer M3 ph = 2.64 (allow more than 2dp but not fewer) Allow for correct ph from their wrong [H + ] If square root forgotten, ph = 5.28 scores 2 for M and M3 (e) M mol OH = ( ) 0.25 = Mark for answer M2 orig mol HX = ( ) 0.74 = Mark for answer M3 mol HX in buffer = orig mol HX mol OH Mark for answer = = Allow conseq on their (M2 M) ([HX] = / = ) If no subtraction, max 3 for M, M2 & M4 (ph = 4.20) If [H + ] = [X ] & used, max 3 for M, M2 & M3 (ph = 2.89) M4 mol X in buffer = mol OH = ([X ] = / = 0.05) May be scored in M5 expression Page 2 of 34

22 M5 [H + ] If use K a = no further marks = OR M6 (= ) If either value of HX or X used wrongly or expression upside down, no further marks ph = 4.48 or 4.49 (allow more than 2dp but not fewer) Do not allow M6 for correct calculation of ph using their [H + ] - this only applies in (d)(iii) - apart from earlier AE [8] M4. (a) (i) addition of small amounts of acid send eqm to left or extra H + removed by reaction with HCO 3 ratio [H 2 CO 3 ]/[HCO 3 ] remains constant hence [H + ] and ph remain const (ii) ph = 7.4 [H + ] = mol dm 3 K a = = = mol dm 3 allow error carried forward mark. Do not penalise twice. (b) (i) moles H + added = = 0.0 (ii) moles ethanoic acid after addition = = 0.6 moles ethanoate ions after addition = = 0.09 Page 22 of 34

23 (iii) [H + ] = = ph = 4.5 [] M5. (a) Z Mark independently. The idea that the solution contains both HA and A (b) ph [HA] = [A ] Accept solution half neutralised. ph = pk a Accept [H + ] = Ka [5] Page 23 of 34

24 M6. (a) This question is marked using levels of response. Refer to the Mark Scheme Instructions for Examiners for guidance on how to mark this question. All stages are covered and the explanation of each stage is generally correct and virtually complete. Answer is communicated coherently and shows a logical progression from stage and stage 2 to stage 3. Steps in stage 3 must be complete, ordered and include a comparison. Level marks All stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies OR two stages are covered and the explanations are generally correct and virtually complete. Answer is mainly coherent and shows a progression from stage and stage 2 to stage 3. Level marks Two stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies, OR only one stage is covered but the explanation is generally correct and virtually complete. Answer includes some isolated statements, but these are not presented in a logical order or show confused reasoning. Level 2 marks Insufficient correct Chemistry to warrant a mark. Level 0 0 marks Indicative Chemistry content Stage : difference in structure of the two acids The acids are of the form RCOOH but in ethanoic acid R = CH 3 whilst in ethanedioic acid R = COOH Stage 2: the inductive effect The unionised COOH group contains two very electronegative oxygen atoms therefore has a negative inductive (electron withdrawing)effect The CH 3 group has a positive inductive (electron pushing) effect Stage 3: how the polarity of OH affects acid strength The O H bond in the ethanedioic acid is more polarised / H becomes more δ + More dissociation into H + ions Ethanedioic acid is stronger than ethanoic acid 6 (b) Moles of NaOH = Moles of HOOCCOO formed = Extended response Page 24 of 34

25 Moles of KMnO 4 = / 000 = Moles of H 2 C 2 O 4 = 5 / = Moles of HOOCCOOH remaining = = K a = [H + ][A ] / [HA] [H + ] = K a [HA] / [A ] [H + ] = ( / V) / ( / V) = ph = log 0 ( ) =.406 =.4 Answer must be given to this precision (c) 5H 2 C 2 O 4 + 6H + + 2MnO 4 2Mn CO 2 + 8H 2 O OR 5C 2 O H + + 2MnO 4 2Mn CO 2 + 8H 2 O Concentration = moles / volume (in dm 3 ) = / 25 = (mol dm 3 ) If : ratio or incorrect ratio used, M2 and M4 can be scored [5] Page 25 of 34

26 M7. (a) [H + ] = or = Allow ( ) M = If [HX] / [X - ] or upside down, or any addition or subtraction lose M & M2. M2 ph = 4.5 (correct answer scores 3) Can score M3 for correct ph conseq to their [H + ], so ph = 5.0 scores one Must be to 2 dp M3 Alternative using Henderson Hasselbach Equation ph = pka log[hx] / X ] = log( ) log( ) Allow ( ) M pka = If [HX] / [X ] or upside down, can only score M2 ph = = 4.5 so ph = 5.0 Must be to 2 dp M3 (b) mol HX after addition (= ) = For HX, if no addition or error in addition (other than AE) (or subsequent extra add or sub) MAX 3 M Page 26 of 34

27 mol X - after subtraction (= ) = 0.25 For X if no subtraction or error in subtraction (other than AE) (or subsequent extra add or sub) MAX 3 M2 [H + ] = ( ) = If errors above in both addition AND subtraction can only score M3 for insertion of their numbers in rearranged expression. One exception, if addition and subtraction reversed then ph =4.58 scores 2 M3 [H + ] = (mol dm 3 ) If [HX] / [X ] upside down, lose M3 & M4 (or next two marks) but can score M5 for correct ph conseq to their [H + ], so if M & M2 correct, ph = 5.09 scores 3. M4 ph = 4.43 Correct use of HX and X values from (d) gives ph= 4.4 and scores 4 If wrong method, e.g. or no use of rearranged K a expression, may score M & M2 but no more. Allow more but not fewer than 2dp here. M5 Page 27 of 34

28 Alternative using Henderson Hasselbach Equation mol acid after addition = = For HX, if no addition or error in addition (other than AE) (or subsequent extra add or sub) MAX 3 M mol salt after addition = = 0.25 For X if no subtraction or error in subtraction (other than AE) (or subsequent extra add or sub) MAX 3 M2 ph = (pka log[hx] / [X ]) = log( ) log(0.266 / 0.25) If errors above in both addition AND subtraction can only score M3 for insertion of their numbers except if addition and subtraction reversed then ph = 4.58 scores 2 M3 ph = M4 ph = = 4.43 If [HX] / [X ] upside down, lose M3 & M4 (or next two marks) but can score M5 for correct ph conseq to their working, so if M & M2 correct, ph = 5.09 scores 3. Allow more but not fewer than 2dp here. M5 [8] M8. (a) (i) [H + ][OH ] OR [H 3 O + ][OH ] Ignore (aq) Must have [ ] not ( ) (ii) (= ) If no square root, CE=0 ph = 6.73 Must be 2dp (iii) [H + ] = 0.36 (= OR ) Mark for working Page 28 of 34

29 Kw = [ OR ] = Allow Mark for answer Ignore units (b) (i) HCOOH HCOO + H + Must have but ignore brackets. OR HCOOH + H 2 O HCOO + H 3 O + Allow HCO 2 or CHOO ie minus must be on oxygen, so penalise COOH (ii) Must have all brackets but allow ( ) Must be HCOOH etc. Allow ecf in formulae from (b)(i) (iii) M Allow HA or HX etc. Allow [H + ] = (Ka [HA]) for M M2 [H + ] = Mark for answer M3 ph = 2.50 allow more than 2 dp but not fewer Allow correct ph from their wrong [H + ] here only If square root shown but not taken, ph = 5.00 can score max 2 for M and M3 (iv) M Decrease Mark M independently M2 Eqm shifts / moves to RHS OR more H + OR K a increases OR more dissociation M3 To reduce temperature or oppose increase / change in temperature Only award M3 following correct M2 Page 29 of 34

30 (c) (i) M If [HX]/[X - ] upside down, no marks M2 (= ) M3 ph = 3.64 allow more than 2 dp but not fewer ph calc NOT allowed from their wrong [H + ] here (ii) M Mol H + added = Mark on from AE in moles of HCl (eg gives ph = 3.42 scores 3) M2 Mol HCOOH = and Mol HCOO = If either wrong no further marks except AE ( ) OR if ECF in mol acid and / or mol salt from (c)(i), can score all 4 M3 If [HX]/[X - ] upside down here after correct expression in (c)(i), no further marks If [HX]/[X - ] upside down here and is repeat error from (c)(i), max 3 (ph = 3.88 after 3.86 in (c)(i)) M4 ph = 3.62 allow more than 2 dp but not fewer ph calc NOT allowed from their wrong [H + ] here [20] Page 30 of 34

31 M9. (a) HCO 3 CO H + or H 2 O + HCO 3 CO H 3 O + Must have equilibrium sign but mark on to (b) Ignore state symbols (b) Acid: Increase in concentration of H + ions, equilibrium moves to the left. Allow H + ions react with carbonate ions (to form HCO 3 - ) Alkali: OH - reacts with H + ions, equilibrium moves to the right (to replace the H + ions) Concentration of H + remains (almost) constant [4] M20. (a) (only) slightly or partially dissociated / ionised Ignore not fully dissociated. Allow low tendency to dissociate or to lose / donate a proton. Allow shown equilibrium well to the left. Otherwise ignore equations. (b) 2CH 3 CH 2 COOH + Na 2 CO 3 2CH 3 CH 2 COONa + H 2 O + CO 2 OR 2CH 3 CH 2 COOH + CO 3 2 2CH 3 CH 2 COO + H 2 O + CO 2 OR CH 3 CH 2 COOH + Na 2 CO 3 CH 3 CH 2 COONa + NaHCO 3 OR CH 3 CH 2 COOH + CO 3 2 CH 3 CH 2 COO + HCO 3 Must be propanoic acid, allow C 2 H 5 COOH. Not molecular formulae. Allow multiples. Ignore reversible sign. Not H 2 CO 3. (c) [OH ] = = M Correct answer for ph with or without working scores 3. Page 3 of 34

32 [H + ] = = OR poh =.62 M2 If 2 missed or used wrongly can only score M3 for correct calculation of ph from their [H + ]. ph = 2.38 M3 Lose M3 if not 2 decimal places: 2.4 scores scores (missing 2) ; 2. scores scores (dividing by 2).8 scores 0. (d) (i) K a = Ignore ( ) here but brackets must be present. Must be correct acid and salt. If wrong, mark part (ii) independently. (ii) M K a = OR with numbers Correct answer for ph with or without working scores 3. Allow HX, HA and ignore ( ) here. May score M in part (i). M2 [H + ] = ( ) or (K a [C 6 H 5 COOH]) (= ( = ) ph = 6.2 may score 2 if correct working shown and they show the square root but fail to take it. But if no working shown or wrong K a = used which also leads to 6.2, then zero scored. M3 ph = 3.06 Must be 2 decimal places ie 3. loses M3. (iii) M [H + ] = = Correct answer for mass with or without working scores 5. Allow 0 4. Page 32 of 34

33 M2 [X ] = Ignore ( ) here. If [HX] / [X ] upside down, can score M plus M4 for M3 = And M5 for g. M4 = M5 Mass (C 6 H 5 COONa) = =.09 g or. g Wrong method, eg using [H + ] 2 may only score M and M5 for correct multiplication of their M4 by 44 (provided not of obviously wrong substance). (e) M CO 2 Allow NOx and SO2. M2 ph (It) falls / decreases If M wrong, no further marks. M3 mark M2 & M3 independently acidic (gas) OR reacts with alkali(ne solution) / OH OR CO 2 + 2OH OR CO 2 + OH CO H 2 O HCO 3 Not forms H 2 CO 3 H 2 SO 3 H 2 SO 4 etc OR H + ions. [7] Page 33 of 34

34 Page 34 of 34

Page 2. M1.(a) [H + ] = or = Allow ( ) M1 1 = If [HX] / [X - ] or M2. M2 1

Page 2. M1.(a) [H + ] = or = Allow ( ) M1 1 = If [HX] / [X - ] or M2. M2 1 M.(a) [H + ] = or =.74 0 5 Allow ( ) M = 3.08 0 5 If [HX] / [X - ] or. upside down, or any addition or subtraction lose M & ph = 4.5 (correct answer scores 3) Can score for correct ph conseq to their [H