AQA Jun 2013 Paper 4 Q3 (with explained solutions) |

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AQA Jun 2013 Paper 4 Q3

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do not write outside the box a this question is about br nsted lowry acids of different strengths state the meaning of the term br nsted lowry acid mark b i write an expression for the acid dissociation constant ka for ethanoic acid mark b ii the value of ka for ethanoic acid is mol dm at c calculate the concentration of ethanoic acid in a solution of the acid that has a ph of marks c the value of ka for chloroethanoic acid clch cooh is mol dm at c c i write an equation for the dissociation of chloroethanoic acid in aqueous solution mark c ii suggest why chloroethanoic acid is a stronger acid than ethanoic acid marks wmp jun chem d p and q are acids x and y are bases the table shows the strength of each acid and base do not write outside the box acids bases strong weak strong weak the two acids were titrated separately with the two bases using methyl orange as indicator the titrations were then repeated using phenolphthalein as indicator the ph range for methyl orange is the ph range for phenolphthalein is for each of the following titrations select the letter a b c or d for the correct statement about the indicator s that would give a precise end point write your answer in the box provided both indicators give a precise end point only methyl orange gives a precise end point only phenolphthalein gives a precise end point neither indicator gives a precise end point d i acid p with base x d ii acid q with base x d iii acid q with base y question continues on the next page mark mark mark turn over wmp jun chem do not write outside the box using a burette cm of mol dm sulfuric acid were added to a conical flask containing cm of mol dm aqueous sodium hydroxide assume that the sulfuric acid is fully dissociated calculate the ph of the solution formed give your answer to decimal places e marks extra space wmp jun chem

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question mark scheme general certificate of education a level chemistry unit kinetics equilibria and organic chemistry june marking guidance mark comments a proton donor or h donor b i ka ch ch coo h cooh or ch coo ch cooh o h b ii m h or mol dm m ch cooh h k a m m mol dm allow c i c ii clch cooh or clch cooh h o clch coo h clch coo h o m cl is more electronegative so withdraws electrons or negative inductive effect of cl m weakens o h bond or o h bond is more polar or reduces negative charge on coo or stabilizes coo more if ka 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 ignore mark for correctly rearranged expression incl h if m wrong no further marks allow allow clch co h and clch co ignore electronegativity ignore chloroethanoic acid has a lower ka value allow cl reduces ve inductive effect of methyl group m m are independent marks ignore h lost more easily mark scheme general certificate of education a level chemistry unit kinetics equilibria and organic chemistry june d i d ii d iii e m mol naoh mol oh m mol h so m mol h added or xs mol h so m xs mol h m for dividing by volume h mol dm m ph mark for answer mark for answer if factor missed completely ph or used wrongly later can score max for m m m m if no use or wrong use of volume lose m and m ie can score for ph no use of vol treat missing as ae score for ph dp penalise more or less if missed vol not used ph scores m m only

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