AQA Jun 2017 Paper 2 Q5 (with explained solutions) |

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AQA Jun 2017 Paper 2 Q5

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Ethanedioic acid (H2C2O4) is a diprotic acid. Beekeepers use a solution of this acid as a pesticide. A student carried out a titration with sodium hydroxide solution to determine the mass of the acid in the solution. The student repeated the titration until concordant titres were obtained. H2C2O4(aq) + 2NaOH(aq)Na2C2O4(aq) + 2H2O(l) The student found that 25.0 cm3 of the ethanedioic acid solution reacted completely with 25.30 cm3 of 0.500 mol dm3 sodium hydroxide solution. Calculate the mass, in mg, of the acid in 25.0 cm3 of this solution. [4 marks] Mass of acid mg The student used a wash bottle containing deionised water when approaching the end-point to rinse the inside of the conical flask. Explain why this improved the accuracy of the titration. [1 mark] Give the meaning of the term concordant titres. [1 mark] Turn overIB/M/Jun17/7404/2 0 5 0 5 . 1 0 5 . 2 0 5 . 3 *09*<br />

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MARK SCHEMEAS-LEVEL CHEMISTRY7404/2JUNE 2017 Question Marking Guidance Mark Comments 05.1 M1 Amount NaOH = 0.02530 x 0.500 = 0.01265 mol M2 Amount acid = 0.006325 mol (i.e. M12) M3 Mr = 90(.0) M4 mass acid = 569 (mg) (allow 567 to 576) (i.e. M2 x M3 in mg) 567-590 = 4 marks 0.567-0.590 = 3 marks Allow ECF at each stage M3 can be scored from use of value of 90(.0) within working M4 should be to at least 2sf. Any individual marks for M1/2/3 should be to at least 2sf (or 90 for M3) 1134-1180 = 3 marks (due to not dividing moles of NaOH by 2) 1.134-1.180 = 2 marks (due to not dividing moles of NaOH by 2 and not converting to mg) 05.2 Idea that it ensures all ethanedioic acid / acid / sodium hydroxide / alkali / reactants are in the mixture / solution / reaction or the idea that it is the transfer of all the acid/alkali alone is not enough the idea that some of the ethanedioic acid / acid / sodium hydroxide / alkali / reactants would be on the sides of the flask 05.3 Titres that are within 0.1 cm3 of each other Units are needed Allow 0.050.15 cm3 Do not allow idea of identical results Allow answers that refer to titres that are within the uncertainty of the burette/apparatus of each other 17 of 26<br />

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