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AQA Specimen Paper 1 Q6
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![14 Ammonium chloride, when dissolved in water, can act as a weak acid as shown by the following equation. NH4 +(aq) NH3(aq) + H+(aq) Figure 1 shows a graph of data obtained by a student when a solution of sodium hydroxide was added to a solution of ammonium chloride. The pH of the reaction mixture was measured initially and after each addition of the sodium hydroxide solution. Figure 1 pH 14 13 12 11 10 9 8 7 6 5 4 9 Volume of NaOH added / cm3 10 11 12 13 [2 marks] . 1 0 6 Suggest a suitable piece of apparatus that could be used to measure out the sodium hydroxide solution. Explain why this apparatus is more suitable than a pipette for this purpose. Apparatus Explanation Barcode Typesetter code<br />
. 2 0 6 . 3 0 6 15 Use information from the curve in Figure 1 to explain why the end point of this reaction would be difficult to judge accurately using an indicator. [2 marks] The pH at the end point of this reaction is 11.8 Use this pH value and the ionic product of water, Kw = 1.01014 mol2 dm6, to calculate the concentration of hydroxide ions at the end point of the reaction. [3 marks] Concentration = mol dm3 Question 6 continues on the next page Barcode Typesetter code Turn over <br />
0 6 16 . 4 The expression for the acid dissociation constant for aqueous ammonium ions is = [+] [ +] The initial concentration of the ammonium chloride solution was 2.00 mol dm3. Use the pH of this solution, before any sodium hydroxide had been added, to calculate a value for Ka [3 marks] . 5 0 6 A solution contains equal concentrations of ammonia and ammonium ions. Use your value of Ka from Question 6.4 to calculate the pH of this solution. Explain your working. (If you were unable to calculate a value for Ka you may assume that it has the value 4.75109 mol dm3. This is not the correct value.) [2 marks] pH= Barcode Typesetter code Ka = mol dm3<br />
17 Turn over for the next question DO NOT WRITE ON THIS PAGE ANSWER IN THE SPACES PROVIDED Barcode Typesetter code Turn over <br />](https://learnah.org/wp-content/uploads/2018/08/Specimen_QP_-_Paper_1_AQA_Chemistry_A-Level_6-0.jpg)
![MARK SCHEMEA-level ChemistrySpecimen paper 1 Marking guidance Mark AO Comments 1 1 1 1 1 1 1 1 1 1 1 1 AO3 1b AO2g AO3 1a AO3 1a AO2h AO2h AO2h AO2f AO2f AO2f AO2h AO2h Allow indicator would not change colour rapidly / with a few drops of NaOH Allow 6.316.33103 (mol dm3) Allow 3.153.161010 (mol dm3) Answer using alternative value M2 pH =log10(4.75109) = 8.32 Allow consequential marking based on answer from 6.4 10 of 15 Question 06.1 06.2 06.3 06.4 Burette Because it can deliver variable volumes The change in pH is gradual / not rapid at the end point An indicator would change colour over a range of volumes of sodium hydroxide [H+] = 10pH = 1.581012 Kw = [H+] [OH] therefore [OH] = Kw / [H+] Therefore, [OH] = 11014/1.581012 = 6.33103 (mol dm3) At this point, [NH3] = [H+] Therefore Ka = [H+]2 +] [NH4 [H+] = 104.6 = 2.51105 Ka = (2.51105)2/2 = 3.151010 (mol dm3) 06.5 When [NH3] = [NH4 Therefore pH =log10(3.151010) = 9.50 +], Ka = [H+] thereforelog Ka =log [H+]<br />](https://learnah.org/wp-content/uploads/2018/08/Specimen_MS_-_Paper_1_AQA_Chemistry_A-Level_6.jpg)