AQA Jan 2013 Paper 4 Q2 (with explained solutions) |

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AQA Jan 2013 Paper 4 Q2

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do not write outside the box in this question give all values of ph to decimal places a the ionic product of water has the symbol kw a i write an expression for the ionic product of water mark a ii at oc the value of kw is mol dm calculate the ph of pure water at this temperature marks a iii at oc a mol dm solution of sodium hydroxide has a ph of calculate a value for kw at this temperature marks question continues on the next page turn over wmp jan chem do not write outside the box b methanoic acid hcooh dissociates slightly in aqueous solution b i write an equation for this dissociation mark b ii write an expression for the acid dissociation constant ka for methanoic acid mark b iii the value of ka for methanoic acid is mol dm at oc calculate the ph of a mol dm solution of methanoic acid marks b iv the dissociation of methanoic acid in aqueous solution is endothermic deduce whether the ph of a solution of methanoic acid will increase decrease or stay the same if the solution is heated explain your answer effect on ph explanation marks extra space wmp jan chem do not write outside the box c the value of ka for methanoic acid is mol dm at oc a buffer solution is prepared containing mol of methanoic acid and mol of sodium methanoate in dm of solution c i calculate the ph of this buffer solution at oc marks extra space c ii a cm sample of mol dm hydrochloric acid is added to the buffer solution in part c i calculate the ph of the buffer solution after this addition marks extra space turn over wmp jan chem

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mark scheme general certificate of education a level chemistry unit kinetics equilibria and organic chemistry january question a i a ii marking guidance h oh or h o oh ignore aq ph mark comments must have not if no square root ce must be dp mark for working mark for answer ignore units must have allow hco penalise cooh but ignore brackets or choo ie minus must be on oxygen so must have all brackets but allow must be hcooh etc allow ecf in formulae from b i allow ha or hx etc allow h ka ha for m mark for answer allow correct ph from their wrong h here only if square root shown but not taken ph can score max for m and m a iii h or kw or allow b i hcooh or hcooh h o hcoo h hcoo h o b ii ka hcoo h hcooh or o h hcoo hcooh b iii m ka h hcooh h m h m ph allow more than dp but not fewer b iv m m m c i m m m c ii m m m m mark scheme general certificate of education a level chemistry unit kinetics equilibria and organic chemistry january mark m independently decrease equm shifts moves to rhs or more h or ka increases or more dissociation to reduce temperature or oppose increase change in temperature h ka x x hx or ph pka log hx x x or ph log ph allow more than dp but not fewer mol h added mol hcooh and mol hcoo h ka x x hx x or ph log ph allow more than dp but not fewer only award m following correct m if hx x upside down no marks ph calc not allowed from their wrong h here mark on from ae in moles of hcl eg x gives ph scores if either wrong no further marks except ae or if ecf in mol acid and or mol salt from c i can score all if hx x upside down here after correct expression in c i no further marks if hx x upside down here and is repeat error from c i max ph after in c i ph calc not allowed from their wrong h here

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 > Equilibrium AS >

1 Dynamic equilibrium and factors affecting equilibrium

AQA > Physical chemistry > Acid base equilibria >

6 Buffer solutions

AQA > Physical chemistry > Thermodynamics >

5 Gibb’s Free Energy

AQA > Physical chemistry > Acid base equilibria >

3 Weak acids

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