AQA Jun 2015 Paper 4 Q9 (with explained solutions) |

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AQA Jun 2015 Paper 4 Q9

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section b answer all questions in the spaces provided do not write outside the box the acid dissociation constant ka for ethanoic acid is given by the expression h ch coo ka ch cooh the value of ka for ethanoic acid is mol dm at c a a buffer solution is prepared using ethanoic acid and sodium ethanoate in the buffer solution the concentration of ethanoic acid is mol dm and the concentration of sodium ethanoate is mol dm calculate the ph of this buffer solution give your answer to decimal places marks wmp jun chem do not write outside the box b in a different buffer solution the concentration of ethanoic acid is mol dm and the concentration of sodium ethanoate is mol dm a sample of hydrochloric acid containing mol of hcl is added to cm of this buffer solution calculate the ph of the buffer solution after the hydrochloric acid has been added you should ignore any change in total volume give your answer to decimal places marks turn over for the next question turn over wmp jun chem

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mark scheme a level chemistry chem june question a b m m m m m m m m h cooh ch ch coo answers or ph correct answer scores mol hx after addition mol x after subtraction h cooh ch ch coo h mol dm ph correct use of hx and x values from a gives ph and scores mark comments allow if hx x or upside down or any addition or subtraction lose m m can score m for correct ph conseq to their h so ph scores one must be to dp for hx if no addition or error in addition other than ae or subsequent extra add or sub max for x if no subtraction or error in subtraction other than ae or subsequent extra add or sub max if errors above in both addition and subtraction can only score m for insertion of their numbers in rearranged expression one exception if addition and subtraction reversed then ph scores if hx x upside down lose m m or next two marks but can score m for correct ph conseq to their h so if m m correct ph scores if wrong method e g or no use of rearranged ka expression may score m m but no more allow more but not fewer than dp here of questio alternative using henderson hasselbach equation answers a m m ph pka log hx x log log pka m ph b m m m mol acid after addition mol salt after addition ph pka log hx x log log m ph m ph mark scheme a level chemistry chem june mar comments allow if hx x or so ph must be to dp upside down can only score for hx if no addition or error in addition other than ae or subsequent extra add or sub max for x if no subtraction or error in subtraction other than ae or subsequent extra add or sub max if errors above in both addition and subtraction can only score m for insertion of their numbers except if addition and subtraction reversed then ph scores if hx x upside down lose m m or next two marks but can score m for correct ph conseq to their working so if m m correct ph scores allow more but not fewer than dp here of

Question 1 2 3 4 5 6 7 8 9 10 11

Topics to revise

AQA > Physical chemistry > Acid base equilibria >

6 Buffer solutions

AQA > Physical chemistry > Thermodynamics >

5 Gibb’s Free Energy

AQA > Physical chemistry > Acid base equilibria >

5 Titration curves and Indicators

AQA > Physical chemistry > Acid base equilibria >

1 Bronsted-Lowry theory and pH

AQA > Physical chemistry > Acid base equilibria >

3 Weak acids

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