AQA Jan 2010 Paper 4 Q2 (with explained solutions) |

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AQA Jan 2010 Paper 4 Q2

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areas outside the box will not be scanned for marking in this question give all values of ph to decimal places a i write an expression for the term ph mark a ii calculate the concentration in mol dm of an aqueous solution of sulfuric acid that has a ph of marks b a student carried out a titration by adding an aqueous solution of sodium hydroxide from a burette to an aqueous solution of ethanoic acid the end point was reached when cm of the sodium hydroxide solution had been added to cm of mol dm ethanoic acid b i write an equation for the reaction between sodium hydroxide and ethanoic acid mark b ii calculate the concentration in mol dm of the sodium hydroxide solution used marks wmp jan chem b iii a list of indicators is shown below areas outside the box will not be scanned for marking indicator thymol blue bromophenol blue litmus cresol purple ph range select from the list the most suitable indicator for the end point of this titration mark b iv suggest why the concentration of sodium hydroxide in a solution slowly decreases when left open to air mark c at k the value of the acid dissociation constant ka for ethanoic acid in aqueous solution is mol dm c i write an expression for the acid dissociation constant ka for ethanoic acid mark c ii calculate the ph of mol dm ethanoic acid at this temperature marks extra space question continues on the next page turn over wmp jan chem areas outside the box will not be scanned for marking c iii calculate the ph of the buffer solution formed when cm of mol dm potassium hydroxide are added to cm of mol dm ethanoic acid marks extra space wmp jan chem there are no questions printed on this page do not write on this page answer in the spaces provided turn over wmp jan chem

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mark comments chem aqa gce chemistry mark scheme january series question part sub part i ii a a b i log h h h so ch cooh naoh ch coona h o or ch cooh oh ch coo h o or log h penalise mark for the answer allow dp or more allow ch co h etc mark for answer if not look back for error penalise allow h o not allow ha etc b b b c ii mol acid or naoh or naoh or cresol purple iii iv naoh reacts with carbon dioxide in the air i ka h ch ch coo cooh allow molecular formulae or minor slip in formulae c ii ka cooh h ch or with numbers h ph can give three ticks here for c ii penalise decimal places c or or iii m mol oh m orig mol ha m mol ha in buffer orig mol ha mol oh m mol a in buffer mol oh m h or cooh ka ch x coo ch or or m ph nb unlike qu c ii this ph mark is not awarded conseq to their h unless following ae can give six ticks for chem aqa gce chemistry mark scheme january series allow ha etc here this can be scored in part c i but doesn t score there mark for or either gives ph mark conseq on their h so gets marks where square root not taken if no subtraction or other wrong chemistry the max score is for m m and m if a is wrong max for m m and m or use of ph pka log ha a mark is for insertion of correct numbers in correct expression for h if ha a upside down lose m m if wrong method e g h ha max for m m and m some may calculate concentrations ha and a and rounding this to gives ph which is ok beware using wrongly instead of gives ph this gets for m m m

Question 1 2 3 4 5 6 7 8 9

Topics to revise

AQA > Physical chemistry > Thermodynamics >

5 Gibb’s Free Energy

AQA > Physical chemistry > Acid base equilibria >

5 Titration curves and Indicators

AQA > Inorganic chemistry > Periodicity AS >

1 Periodicity

AQA > Physical chemistry > Acid base equilibria >

6 Buffer solutions

AQA > Physical chemistry > Acid base equilibria >

1 Bronsted-Lowry theory and pH

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