CIE Nov 2016 v2 Paper 5 Q1

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PMT Titrations using ethylenediaminetetraacetic acid (EDTA) can be used to determine the concentration of metal ions in solution, such as Zn2+(aq). A solution of EDTA is usually prepared from the hydrated disodium salt, Na2H2Y.2H2O. The anion of EDTA is H2Y2, where Y represents the organic part of the ion. The equation for the reaction between Zn2+(aq) and EDTA is shown. Zn2+(aq) + H2Y2(aq)ZnY2(aq) + 2H+(aq) The indicator for the reaction is Solochrome Black, which changes colour at the endpoint from purple to blue. The indicator only works at pH 10, so a buffer solution is added to the metal ion solution to maintain the pH. (a) Explain why the pH would change during the titration if the buffer were not present.[1] (b) You are to plan a titration experiment to determine the concentration of zinc ions in a solution of zinc sulfate of concentration approximately 0.1 mol dm3. You are provided with the following materials. 20.0 g of hydrated disodium EDTA, Na2H2Y.2H2O (Mr = 372.2) aqueous zinc sulfate of approximate concentration 0.1 mol dm3 buffer solution, pH 10 Solochrome Black indicator solution (i) Name three pieces of volumetric apparatus you would use, with their capacities in cm3. 123[2]UCLES 2016 9701/52/O/N/16<br /> (ii) Calculate the mass of hydrated disodium EDTA that would be required for the preparation of a standard solution of concentration 0.100 mol dm3, using the apparatus you have speci ed in (i). PMT (iii) Describe how you would prepare this standard solution for use in your titration. mass of hydrated disodium EDTA =g [1][2] (iv) After you have performed a rough titration, how would you ensure that your next titration is accurate?[1] (v) How would you ensure that your titration result is reliable?[1]UCLES 2016 9701/52/O/N/16 [Turn over<br /> PMT (c) The term hard water is used to describe water containing the dissolved metal ions, Ca2+(aq) and Mg2+(aq). Both of these metal ions react with EDTA anions, H2Y2. Ca2+(aq) + Mg2+(aq) + 2H2Y2(aq)CaY2(aq) + MgY2(aq) + 4H+(aq) In an experiment to determine the concentration of each of these metal ions, two separate titrations with EDTA need to be performed. For titration 1, a 25.0 cm3 sample of hard water is titrated with 0.0100 mol dm3 EDTA solution using Solochrome Black solution as indicator. For titration 2, another 25.0 cm3 sample of the same hard water isrst treated with excess 2 mol dm3 NaOH(aq) which precipitates all of the Mg2+(aq) ions as Mg(OH)2(s). After this treatment, no Mg2+(aq) ions remain in solution, leaving only dissolved Ca2+(aq) ions in solution. This solution is then titrated with 0.0100 mol dm3 EDTA solution using Solochrome Black solution as indicator. The following information gives some of the hazards associated with the chemicals used in the procedure. Sodium hydroxide Solutions equal to or more concentrated than 0.5 mol dm3 are classi ed as corrosive. Solochrome Black Solid Solochrome Black is classi ed as health hazard and is irritating to eyes, respiratory system and skin. All solutions are made up in ethanol and so are classi ed asammable and health hazard. (i) Identify one hazard that must be considered when planning the experiment and describe a precaution, other than eye protection, that should be taken to keep risks from this hazard to a minimum. hazard:precaution:[1]UCLES 2016 9701/52/O/N/16<br /> (ii) Results obtained from this experiment are shown. titre 1, 22.70 cm3 titre 2, 16.60 cm3 Use the results of the titrations to determine the concentrations of Ca2+(aq) and Mg2+(aq) in the hard water. PMT concentration of Ca2+(aq)mol dm3 concentration of Mg2+(aq)mol dm3 [4] [Total: 13]UCLES 2016 9701/52/O/N/16 [Turn over<br />

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