M.C [] M.B [] M.D [] M4.B [] M5.D [] M6.D [] M7.A [] M8.A [] M9.C [] M0.D [] M.C [] M. B [] M. C [] Page of
M4. (a) Burette Because it can deliver variable volumes (b) The change in ph is gradual / not rapid at the end point An indicator would change colour over a range of volumes of sodium hydroxide Allow indicator would not change colour rapidly / with a few drops of NaOH (c) [H + ] = 0 ph =.58 0 K w = [H + ] [OH ] therefore [OH ] = K w / [H + ] Therefore, [OH ] = 0 4 /.58 0 = 6. 0 (mol dm ) Allow 6. 6. 0 (mol dm ) (d) At this point, [NH ] = [H + ] Therefore K a [H + ] = 0 4.6 =.5 0 5 K a = (.5 0 5 ) / =.5 0 0 (mol dm ) Allow.5.6 0 0 (mol dm ) (e) When [NH ] = [NH 4+ ], K a = [H + ] therefore log K a = log [H + ] Answer using alternative value Therefore ph = log 0 (.5 0 0 ) = 9.50 M ph = log 0 (4.75 0 9 ) = 8. Allow consequential marking based on answer from part (d) [] M5.B [] Page of
Allow CH CO H, CH CO H +, CH C + (OH) M6. (a) CH COOH + H O CH COO - + H O + OR CH COOH CH COO - + H + Must show - Allow CH CO H, CH CO Ignore state symbols (b) (CH COOH + HNO CH COOH + + NO - ) IGNORE (c) (i) ( new [HNO ] = [H + ] = 0.05 ) M [H + ] = 8.() 0 - (mol dm - ) OR new[hno ] = M ph = log M OR.08 Must be dp Allow correct ph conseq to their [H + ] concentration (ii) M mol NaOH (= 50 0-0.008) = 5.40 0-4 M Subtraction of M from moles of HNO (.5 0 - or conseq from c(i)) Excess mol H + = 7.0 0-4 M allow ecf for subtraction of mol If no subtraction, no further marks M [H + ] = = 4.7 0 - M if no use of volume, no further marks (ph=.5) If incorrect volume used, can score M4 M4 ph = -log M OR. M4 Allow. Must be dp Page of
(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 M K a = or with numbers or with HA M [H + ] = [ (.74 0-5 0.05)] = 4.66 0-4 Mark for answer M4 ph =. Must be dp Allow correct ph conseq to their [H + ] concentration (ph =.8 can score M, M and M4) (ii) Sodium ethanoate Ignore formula Allow sodium acetate (iii) M [H + ] =.45 0-5 Accept.445 0-5 or.4 0-5 M If M incorrect CE=0 Inclusion of 0.05 in calculation can only score M M.(0) Ignore units.4 0-5 gives.4 (e) M (Electronegative) chlorine withdraws electrons Allow Cl has negative inductive effect M Stabilises/reduces charge on COO- OR weakens O-H bond OR makes O-H more polar Ignore chloroethanoic acid dissociates more readily Mark independently Page 4 of
(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 M K a value for strong acids tends to infinity/is very large OR can t divide by zero in K a [0] M7. (a) log [H + ] ecf if [ ] wrong and already penalised 4.57 0 allow 4.6 0 ignore units (b) (i) K a = allow HA etc not but mark on If expression wrong allow conseq units in (ii) but no other marks in (ii) (ii) = If use 4.6 0 K a =.4() 0 4 and pka =.85 =.9 0 4 allow.9.4 0 4 mol dm (iii) pk a =.86 Penalise dp of final answer < or > in ph once in paper Page 5 of
(c) (i) 0.480 = 0.044 or.4(4) 0 Mark is for answer (M) (ii) 0.50 = 0.006 or 6. 0 Mark is for answer (M) (iii) 0.044 (0.006) =.80 0 M is for (i) (ii) If x missed, CE i.e. lose M and the next mark gained (iv).80 0 = 0.075 (0.08) M4 is for answer If vol is not 48 0 (unless AE) lose M4 and next mark gained If multiply by 48 - this is AE - i.e. lose only M4 If multiply by 48 0 this is AE - i.e. lose only M4 (v) 0 4 / 0.075 (0 4 / 0.08) M5 for K w /[OH ] (=.66 0 ) (=.6 0 ) or poh lose both M5 and M6 or poh =.46 (or poh =.40) If no attempt to use K w or poh ph =.57 (.58) M6 Allow M6 conseq on AE in M5 if method OK [] M8. (a) (i) B C A (ii) cresolphthalein or thymolphthalein Page 6 of
(b) ph = -log[h + ] K a = or [H + ] = [A ] [H + ] =.74 0 5 0.5 (or.6 0 ) ph =.79 (penalise dp or more than dp once in the qu) [8] M9. (a) (i) [H + ][OH ] log [H + ] (ii) [H + ] = [OH ] (iii) (.0 0 ) 0.5 =.0 0 (iv) [H + ] = (= 4.0 0 ) ph = 0.40 (b) (i) K a = [H + ][CH CH COO - ] [CH CH COOH] = [H + ] [CH CH COOH] [H + ] = (.5 0 5 ) 0.5 (=.0 0 ) ph =.89 Page 7 of
(c) (i) (50.0 0 ) 0.5 = 6.5 0 (ii) (6.5 0 ) (.0 0 ) = 5.5 0 (iii) mol salt formed =.0 0 [H + ] = K a [CH CH COOH] [CH CH COO ) = (.5 0 5 ) (= 7.088 0 5 ) ph = 4.5 [6] M0. (a) ph on the y-axis, volume of alkali on the x-axis If axes unlabelled use data to decide that ph is on y-axis. Uses sensible scales Lose this mark if plotted paths do not cover half of the paper. Lose this mark if the graph plot goes off the squared paper. Labels the axes Allow mark for axes labelled ph and volume. Plots all of the points correctly Line through the points is smooth and has the correct profile Ignore 0 5 cm section of the graph. Lose this mark if graph is kinked or not a single line. Line ignores the point at cm Lose this mark if point clearly not treated as an anomaly. (b) (i) 4.4 cm ± 0. If no answer in (i) allow answer written on the graph. Allow this answer only. Do not penalise precision. Page 8 of
(ii). cm ± 0. If no answer in (ii), allow answer written on the graph. Allow answer to (i) divided by. Do not penalise precision. (iii).9 ± 0. If no answer in (iii), allow answer written on the graph. Consequential marking from (ii) Lose this mark if answer not given to dp. (c) pk a = log K a or K a = 0 x, where x = (answer to b(iii)).6 0 4.7 to 4. gives K a = 7.9 0 5 to.0 0 4 Consequential marking from b(i). Correct answer without working scores mark only. Do not penalise precision. (d) Methanoic acid Consequential marking from (c). pk a =.7 gives methanoic acid. pk a = 4. gives ethanoic acid. No lucky guesses candidates must apply answer from (c). Do not allow answers based on data given in (f). (e) Error in using pipette is 0.% and Error in using burette is 0.5 00 / (answer to b(i)) Using 4.4 for burette gives 0.6% Do not penalise precision. Allow if errors are given without working. Lose mark if the burette error is not calculated on b(i). If the error being calculated is not stated, allow if the calculations are in the same order as in the question (pipette, burette). (f) Difference is.6 0 4.6 0 4 = 0.4 0 4 Allow consequential answer from (c). Do not penalise precision. 0.4 00 /.6 is a % error Correct final answer without working scores mark. Using.9 0 4 gives 0. 0 4 and 8.8%. (g) Calibrate meter or thermostat the mixture or maintain constant temperature Do not allow repeat experiment. Page 9 of
(h) Mixture is a buffer [6] M. (a) (i) - log[h + ] penalise missing [ ] here and not elsewhere (ii) [H + ][OH ] Allow ( ) brackets, but must have charges (iii) Mark independently from a(ii) [H + ] = 0 -.7 =.905 0 4 If wrong no further mark K w =.905 0 4 0.54 = = (.9.94) 0 5 (b) (i) Ka = Must have charges and all brackets, allow ( ) Acid/salt shown must be CH COOH not HA and correct formulae needed (ii) In ph values penalise fewer than sig figs each time but allow more than dp For values above 0, allow sfs - do not insist on dp K a = Allow HA ([H + ] =.75 0 5 0.54 =.695 0 6 =.70 0 6 ) If shown but not done gets ph = 5.57 (scores ) [H + ] =.64 0 Allow mark for ph conseq to their [H+] here only ph =.78 or.79 Page 0 of
(c) (i) In ph values penalise fewer than sig figs each time but allow more than dp For values above 0, allow sfs - do not insist on dp M Initially mol OH = (0 0 ) 0.54 and mol HA = (0 0 ) 0.54 or mol OH =.54 0 and mol HA =.08 0 M [H + ] = K a or with numbers Allow Henderson Hasselbach ph = pk a + log M mol ethanoic acid left = (mol ethanoate ions) =.54 0 K a = [H + ] or ph = pk a scores M, M and M If either mol acid in mixture or mol salt wrong - max for M and M Any mention of [H + ] - max for M and M M4 ph (= - log.75 0 5 ) = 4.76 or 4.757 Not 4.75 If no subtraction (so mol ethanoic acid in buffer = original mol) ph = 4.46 scores for M and M If [H+] used, ph =.0 scores for M and M (ii) In ph values penalise fewer than sig figs each time but allow more than dp For values above 0, allow sfs - do not insist on dp M XS mol KOH (= (0 0 ) 0.54) =.08 0 If no subtraction: max for correct use of volume No subtraction and no use of volume scores zero If wrong subtraction or wrong moles Can only score M and M for process Page of
M [OH - ] =.08 0 = 0.05() Mark for dividing their answer to M by correct volume (method mark) If no volume or wrong volume or multiplied by volume, max for M and M process M [H + ] = (=.948 0 to.95 0 ) or poh =.9 Mark for K w divided by their answer to M If poh route, give one mark for 4 poh M4 ph =.7() Allow sf but not.70 If no subtraction and no use of volume (ph =.79 scores zero) If no subtraction, max for correct use of volume, (60cm ) (ph =.0 scores ) If volume not used, ph =.49 (gets ) If multiplied by vol, ph = 0.7 (gets ) [6] M. (a) (i) log[h + ] (ii) [H + ][OH ] Penalise missing [ ] here and not elsewhere (b) (i) [H + ] =.4 0 7 ph = 6.6 Penalise fewer than sig figs but allow more than dp (ii) [H + ] = [OH ] Page of
(iii) M [H + ] = K w /[OH ] if upside down or CE, allow M only for correct use of their [H + ] M (= 5.48 0 4 /0.40) =.9 0 M ph =.4() not.40 (AE from.407) Penalise fewer than sig figs but allow more than sfs For values above 0, allow sfs - do not insist on dp. For values below, allow dp do not insist on sig figs Not allow ph = 4 poh but can award M only for ph =.(46) Can award all three marks if pk w =.6 is used (c) M mol NaOH = mol OH = (0 0 ) 0.0 = 6.0 0 mark for answer M mol H SO 4 = (5 0 ) 0.5 =.75 0 mark for answer M mol H + = (5 0 ) 0.5 = 7.5 0 OR XS mol H SO 4 = 0.75 0 if factor of missed or used wrongly, CE - lose M and next mark gained. In this case they must then use K w to score any more. see examples below M4 XS mol H + =.5 0 M5 [H + ] = (.5 0 ) (000/55) = 0.07 M6 ph =.56 if no use or wrong use of volume, lose M5 and M6 except if 000 missed AE (ph = 4.56) Penalise fewer than sig figs but allow more than sfs For values above 0, allow sfs - do not insist on dp. For values below, allow dp do not insist on sig figs [4] Page of
M. (a) M [H + ] = 0.070 M ph =.77 dp Allow M for correct ph calculation from their wrong [H + ] for this ph calculation only (b) (i) Ignore Penalize missing [ ] here and not elsewhere Allow HA instead of HX (ii) M [H + ] = 0.79 OR.68 0 If [H + ] wrong, can only score M M OR Allow HA instead of HX M K a =.09 0 5 sfs min (allow.0 0 5 if.68 rounded to.6) Ignore units If [HX] used as (0.0850.6 0 ) this gives K a =.5 0 5 (0.006) /0.085 =.0 0 5 scores for AE (c) M mol OH (= (8. 0 ) 0.550 ) =.0() 0 or 0.00() Mark for answer M Mol H + (= (5.0 0 ) 0.60 ) =.55 0 or 0.055 Mark for answer M excess mol OH = 5.5() 0 Allow conseq for M M If wrong method e.g. no subtraction or use of can only score max of M, M, M and M4. Page 4 of
M4 [OH ] = 5.5 0 [ = 0.0878 (0.087)] OR [OH ] = 5.5 0 = 0.0870() (M M) / vol in dm mark for dividing by volume (take use of 6. without 0 as AE so 9.94 scores 5) If no use or wrong use of vol lose M4 & M6 Can score M5 for showing (0 4 / their XS alkali) M5 [H + ] = =.47 0 OR =.49 0 OR poh =.06 If no use or wrong use of K w or poh no further marks M6 ph =.9(4) allow sf If vol missed score max 4 for.7(4) If acid alkali reversed max 4 for ph =.06 Any excess acid max 4 [] M4. (a) Correct orientation of graph (ph on y-axis) Scale plotted points cover at least half the grid and y-axis should start at ph 4 All points plotted correctly + / one small square. Curve of best fit drawn correctly Allow some leniency here with a complex graph it is important that the section between ph 8.5 and 9.7 is close to linear. Lose this mark if the line is pulled towards the anomaly at.0 cm. Lose this mark if first point at ph 5. is treated as an anomaly. Do not accept doubled lines but allow some slight discontinuity where the curve changes direction. (b).6-.9 (cm ) only Do not mark consequentially to student s graph. Page 5 of
(c) pk a = value of ph related to part (b) M Mark consequentially on student s graph ideally 9.0-9. Do not penalise precision of answer. K a = 0 pka M Ideally.0 0 9 to 7.9 0 0 Ignore precision of answer but lose M for significant figure here. (d) ph 8.7 Ineffective stirring / swirling of the mixture Both points needed for this mark. Do not allow ph 5. Do not allow overshooting (at cm addition). (e) Take more ph readings around the end-point / add smaller volumes of NaOH near the end-point Do not allow use a more accurate / reliable ph meter / probe. Do not allow the use of a thermostatted mixture. [9] M5. (a) Proton donor or H + donor (b) (i) If K a wrong, can only score M below. Must be ethanoic acid not HA Must have square brackets (penalise here only) but mark on in (b) (ii). (ii) M [H + ] = 0.69 OR.04 0 (mol dm ) Page 6 of
M Ignore ( ) Mark for correctly rearranged expression incl [H + ] M If M wrong no further marks. M4 = 0.8 (mol dm ) Allow 0.9 0.4 (c) (i) ClCH COOH ClCH COO + H + OR ClCH COOH + H O ClCH COO + H O + (ii) Allow Allow ClCH CO H and ClCH CO M Cl is (more electronegative so) withdraws electrons OR negative inductive effect of Cl Ignore electronegativity. Ignore chloroethanoic acid has a lower K a value. Allow Cl reduces +ve inductive effect of methyl group. M Weakens O H bond OR O H bond is more polar OR reduces negative charge on COO OR stabilizes COO (more) M & M are independent marks. Ignore H + lost more easily. (d) (i) A (ii) (iii) C D Page 7 of
allow RHS as C H 5 NH OH (e) M Mol NaOH = mol OH = (9.6 0 ) 0.70 =.4() 0 Mark for answer. M Mol H SO 4 = (6.4 0 ) 0.550 =.45() 0 Mark for answer. M Mol H + added = (.45 0 ) =.90(4) 0 OR XS mol H SO 4 = 7.46(4) 0 If factor missed completely (ph =.05) or used wrongly later, can score max 4 for M, M, M5 & M6 M4 XS mol H + = 0.049() M5 For dividing by volume [H + ] = 0.049() (000 / 46.0) = 0.4 0.5 mol dm If no use or wrong use of volume lose M5 and M6 ie can score 4 for ph =.8 (no use of vol) Treat missing 000 as AE ( ) & score 5 for ph =.49 M6 ph = 0.49 dp (penalise more or less). If missed & vol not used, ph =.9 scores M & M only. [8] M6. (a) Proton acceptor + (b) (i) CH CH NH + H O CH CH NH + OH allow eq with or without + allow C H 5 NH and C H 5 NH (plus can be on N or H or ) Page 8 of
(ii) Mark independently of (b)(i) Allow Ethylamine is only partly/slightly dissociated OR Ethylamine is only partly/slightly ionized reaction/equilibrium lies to left or low [OH ] OR little OH formed OR little ethylamine has reacted Ignore not fully dissociated or not fully ionized Ignore reference to ionisation or dissociation of water (c) M Ethylamine If wrong no marks in (c) M alkyl group is electron releasing/donating OR alkyl group has (positive) inductive effect M increases electron density on N(H ) OR increased availability of lp OR increases ability of lp (to accept H(+)) Mark M is independent of M (d) CH CH NH Cl Or any amine hydrochloride allow name (ethylammonium chloride or ethylamine hydrochloride) or other halide for Cl or a strong organic acid NOT NH 4 Cl (e) Mark independently of (d) Extra H + reacts with ethylamine or OH Or makes reference to Equilibrium (in (b)(i)) with amine on LHS Page 9 of
OR CH CH NH + H + CH CH NH + OR H + + OH H O Equilibrium shifts to RHS OR ratio [CH CH NH + ]/[ CH CH NH ] remains almost constant [9] M7. (a) (i) G (ii) (iii) F H (b) (i) cresol purple (ii) yellow to red both colours needed and must be in this order (iii) yellow or pale yellow Not allow any other colour with yellow [6] M8. (a) Idea that over time / after storage meter does not give accurate readings Do not accept to get an accurate reading without further qualification. Allow temperature variations affect reading. (b) Allow without (aq) symbols. Need at least one set of square brackets around complex ions (c) ph = log [H + ] [H + ] = 0.040 Do not penalise precision of [H + ] Correct answer scores M and M. Page 0 of
K a = (0.040) / 0. = 5.75 0 - or 5.76 0 - Correct answer without working loses M and M. Allow 7.58 0 - Answer, even if incorrect, given to sig figs (d) Oxygen (in the air) / O Ignore air or the atmosphere or chemicals in soil. List principle. (e) 4.0 6.9 Do not penalise precision. [7] Page of
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