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Edexcel Jun 2015 Paper 4 Q13
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![Correct Answer Question Number 13 (a) (ii) Reject Mark 1st scoring point: Propanoate ions present (at equivalence point) OR Potassium propanoate present (at equivalence point) (1) 2nd scoring point: Propanoate (ions) react with water / propanoate (ions) are hydrolysed by water / CH3CH2COO ions react with water ALLOW propanoate ions react with H+ (from water) / the salt reacts with water (molecules) (1) 3rd scoring pointconsequential on 2nd scoring point being awarded: Forming hydroxide ions/ leaves excess of hydroxide ions / produces OH / forming OH / forming KOH / [OH] > [H+] (1) NOTEthe equation: CH3CH2COO + H2OOH + CH3CH2COOH OR CH3CH2COOK + H2OKOH + CH3CH2COOH scores ALL THREE MARKS NOTE Just weak acidstrong base titration scores (1) only<br />
Reject Mark Question Number 13 (a) (iii) Correct Answer [FIRST, CHECK THE FINAL ANSWER IF ANSWER pH = 12(.02), award 5 marks] Moles of acid used = 25/1000 x 0.024 OR moles of acid used = 6 x 104 (mol) and Moles of alkali added = 40/1000 x 0.032 OR Moles of alkali added = 1.28 x 103 (mol) (1) Moles of excess alkali = 1.28 x 1036 x 104 OR Moles of excess alkali = 6.8 x 104 (mol) (1) [OH] = 6.8 x 104 /(65/1000) = 0.01046 (mol dm3) (1) Allow TE from incorrect moles of acid or alkali, provided the alkali moles are in excess [H+] = 1 x 1014 / 0.01046 = 9.56 x 1013 (mol dm3) (1) Allow TE from incorrect moles of excess alkali or the candidates value of [OH]. Must use Kw value here to get [H+] pH = log 9.56 x 1013 = 12(.02) (1) Can get M4 and M5 using pH + pOH = 14 Allow TE from incorrect [H+] for M5, but their CQ pH must > 7 IGNORE S.F. EXCEPT 1 SF<br />
NOTE If fail toby 0.065 dm3, then pH = 10.8 scores 4 marks. Other answers to look for if M1 and M2 have been awarded, but division by an incorrect value for the total volume of the mixture, then each of the following would score 4 overall as shown. Ifby 0.025 dm3, no M3 pH = 12(.43) scores 4 marks. Ifby 0.040 dm3, no M3 pH = 12(.23) scores 4 marks. Ifby 0.015 dm3, no M3 pH = 12(.66) scores 4 marks.<br />
Question Number 13 (b) Reject Mark Correct Answer No, as T increases eqm moves to RHS / Kw increases / favours RHS / Stotal increases (1) So [H+] ions increases / more H+ ions [H+] > 1 x 107 (1) Hence pH < 7 / pH decreases (1) OR reverse argument for a decrease in temperature NOTE If answer given is Yes (i.e. candidate thinks that the pH of pure water is always 7.0), then max (1) for stating that equilibrium shifts to the right when temperature increases (since reaction is endothermic in the forward direction) NOTE If says Kw decreases as T increases, then max (1) for a completely logical CQ argument mentioning the effect on [H+] (decreasing) and pH (increasing) (TOTAL FOR QUESTION 13 = 14 marks)<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_MS_-_Unit_4_Edexcel_Chemistry_A-level_13-0.jpg)
