AQA Jun 2012 Paper 4 Q3 (with explained solutions) |

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AQA Jun 2012 Paper 4 Q3

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do not write outside the box a this question is about several br nsted lowry acids and bases define the term br nsted lowry acid mark b three equilibria are shown below for each reaction indicate whether the substance immediately above the box is acting as a br nsted lowry acid a or a br nsted lowry base b by writing a or b in each of the six boxes b i ch cooh h o ch coo h o b ii ch nh h o ch nh oh b iii hno h so h no hso mark mark mark c a cm sample of mol dm hydrochloric acid was placed in a beaker distilled water was added until the ph of the solution was calculate the total volume of the solution formed state the units marks extra space question continues on the next page turn over wmp jun chem do not write outside the box d at k the value of the acid dissociation constant ka for the weak acid hx in aqueous solution is mol dm d i calculate the value of pka for hx at this temperature give your answer to decimal places mark d ii write an expression for the acid dissociation constant ka for the weak acid hx mark d iii calculate the ph of a mol dm solution of hx at this temperature give your answer to decimal places marks extra space wmp jun chem do not write outside the box e an acidic buffer solution is formed when cm of mol dm aqueous sodium hydroxide are added to cm of mol dm aqueous hx the value of ka for the weak acid hx is mol dm calculate the ph of this buffer solution at k give your answer to decimal places marks extra space turn over wmp jun chem

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mark scheme general certificate of education a level chemistry unit kinetics equilibria and organic chemistry june question a b i b ii b iii c marking guidance proton donor or h donor b b a a b a h or m m mol hcl mark additional guidance allow donator both need to be correct to score the mark both need to be correct to score the mark both need to be correct to score the mark mark for working units and answer tied lose m if total given as cm ignore vol added cm after correct answer must be dp must have all brackets but allow allow ha etc no mark for pka allow h ka ha for m mark for answer allow for correct ph from their wrong h if square root forgotten ph scores for m and m m vol dm or cm allow dm or cm d i d ii ka h x hx ignore h xh but this may score m in d iii d iii m ka h xh or with numbers h m m ph allow more than dp but not fewer mark scheme general certificate of education a level chemistry unit kinetics equilibria and organic chemistry june e m mol oh mark for answer m orig mol hx mark for answer m mol hx in buffer orig mol hx mol oh hx m mol x in buffer mol oh x m h ka x x hx or x x ph or m allow more than dp but not fewer mark for answer allow conseq on their m m if no subtraction max for m m m ph if h x used max for m m m ph may be scored in m expression if use ka h xh no further marks if either value of hx or x used wrongly or expression upside down no further marks do not allow m for correct calculation of ph using their h this only applies in d iii apart from earlier ae

Question 1 2 3 4 5 6 7 8

Topics to revise

AQA > Physical chemistry > Acid base equilibria >

1 Bronsted-Lowry theory and pH

AQA > Physical chemistry > Acid base equilibria >

6 Buffer solutions

AQA > Organic Chemistry > Aromatic chemistry >

2 Electrophilic substitution

AQA > Physical chemistry > Acid base equilibria >

3 Weak acids

AQA > Physical chemistry > Acid base equilibria >

5 Titration curves and Indicators

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