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CIE Nov 2014 v1 Paper 5 Q2
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![PMT The acid dissociation constant, Ka, of a weak monoprotic acid, HA, is to be determined from the measurement of the pH change that occurs when it is titrated with an aqueous solution of sodium hydroxide. 2.70 g of HA was dissolved in distilled water to make exactly 250.0 cm3 of solution. 25.00 cm3 of the solution was pipetted into a beaker. The pH of the acid in the beaker was measured and recorded in the table below. A burette was thenlled with aqueous sodium hydroxide and the 25.00 cm3 of HA was titrated by adding volumes of the aqueous sodium hydroxide to the beaker as indicated in the table below. After each addition the pH was measured and the value recorded. volume of sodium hydroxide added / cm3 pH measured 0.00 2.00 4.00 8.00 12.00 16.00 20.00 24.00 28.00 30.00 32.00 36.00 2.41 2.75 3.09 3.46 3.52 3.96 4.20 4.50 5.05 7.00 11.55 12.00UCLES 2014 9701/51/O/N/14<br />
(a) Plot a graph to show how the pH of the mixture changes with the volume of added aqueous sodium hydroxide as shown in the table. Draw a smooth curve, using the plotted points on your graph, to produce a titration curve for the addition of aqueous sodium hydroxide to the acid HA. PMT pH 13.0 12.0 11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 volume of sodium hydroxide added / cm3UCLES 2014 9701/51/O/N/14 [2] [Turn over<br />
PMT (b) Circle any points on the graph that are anomalous and suggest a reason why this might occur.[2] (c) What would be a suitable range of pH values in which an indicator would change colour to identify the end point of this neutralisation?[1] (d) 30.00 cm3 of aqueous sodium hydroxide is required to neutralise 25.00 cm3 of HA and the equation for the neutralisation is shown. NaOH + HANaA + H2O (i) Excluding water, state the three ions or molecules that will be present in the highest concentration when 15.00 cm3 of aqueous sodium hydroxide has been added to 25.00 cm3 of HA.[1] (ii) State and explain how the concentrations of these ions or molecules compare.[2]UCLES 2014 9701/51/O/N/14<br />
(e) Use your graph to determine the pH obtained when 15.00 cm3 of aqueous sodium hydroxide is added to 25.00 cm3 of HA. Use this pH to determine the value of Ka for HA. PMT (f) (i) Use your answer to (e) and the initial pH of HA from the table to calculate the concentration of HA in mol dm3. [3] (ii) Calculate the initial concentration of HA, in g dm3, and use this together with your answer to (f)(i) to calculate the relative molecular mass, Mr, of HA. (Remember that 2.70 g of HA was dissolved in distilled water to make exactly 250.0 cm3 of solution.) [2]UCLES 2014 9701/51/O/N/14 [1] [Turn over<br />
PMT 10 (g) Even if the experiment is done very carefully with very accurate apparatus, the answer obtained for the molecular mass of HA is likely to be subject to error. Suggest why.[1] [Total: 15]UCLES 2014 9701/51/O/N/14<br />](https://learnah.org/wp-content/uploads/2018/08/November_2014_v1_QP_-_Paper_5_CIE_Chemistry_A-level_2-0.jpg)
![Question (a) Page 4 Mark Scheme Syllabus Paper Cambridge International A LevelOctober/November 2014 9701 51 Expected Answer Mark All plotted points correctly drawn The best fit line should pass through or lie close to the first 10 points. (If all points do not lie on the line then the net deviation of the non-anomalous points on each side of the best fit line must be approximately the same.) (b) The point at 12.00 cm3 is an anomaly. (ECF from incorrect anomalous point.) The sodium hydroxide has not been properly mixed with the acid OR insufficient sodium hydroxide was added. (c) Indicator range between 6.5 and 11 for a minimum of 1 pH unit change. (d) (i) Any two of the following: Na+, A, HA (ii) They will all have (nearly) the same concentration OR A > Na+ > HA Half of the HA has reacted with/been neutralised by / used up by the NaOH (e) Reads correct value of pH from the graph drawn Gives correct expression for Ka Calculates Ka. (f) (i) [H+] = 0.00389 mol dm3 OR [H+] = [A] ECF as (ans to [H+])2/(ans to (e)) Calculates [HA] correctly based on the pH read from the graphCambridge International Examinations 2014 1 1 1 1 1 1 1 1 1 1 1 [2] [2] [1] [1] [2] [3] [2]<br />
Page 5 Mark Scheme Syllabus Paper Cambridge International A LevelOctober/November 2014 9701 51 Question Expected Answer (ii) Conc of HA = 42.7 = 10.8 g dm3 Relative molecular mass of HA = 10.8/ 2(f)(i) (g) Any appropriate error discussion e.g.: the H+ from the water has been ignored, (cid:127) many readings / measurements are taken each of which will have an error., (cid:127) (cid:127) no pH reading was taken at 15.00 cm3, (cid:127) H+ is not exactly equal to A temperature varies during titration, (cid:127) graph drawn by hand is not very accurate, (cid:127) experiment not repeated. Mark [1] [1] Total 15Cambridge International Examinations 2014<br />](https://learnah.org/wp-content/uploads/2018/08/November_2014_v1_MS_-_Paper_5_CIE_Chemistry_A-level_2-0.jpg)
