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OCR A Specimen Paper 3 Q2
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![2 This question looks at ions and complexes. (a)* You are provided with two boiling tubes containing solutions of the same ionic compound. The compound contains one cation and one anion from the lists below.cations: Fe2+, Mn2+, NH4 +anions: Cl, CO3 2, SO4 2 Solutions of common laboratory reagents are available. Plan a series of tests that you could carry out on the samples to identify the ionic compound. Your tests should produce at least one positive result for each ion. For each test,include details of reagents, relevant observations and equationsexplain how your observations allow the ions to be identified. You may include flowcharts or tables in your answer. SpecimenOCR 2014 H432/03<br />
[6] (b) The dissociation of water is measured by the ionic product of water, Kw. The value of Kw varies with temperature as shown in the graph below. Kw / 1014 mol2 dm6 5.0 4.0 3.0 7.0 6.0 Specimen temperature / C 0.0 2.0 1.0 10 20 30 Calculate the pH of water at body temperature, 37 C. 40 50 60OCR 2014 H432/03 pH =[3] Turn over<br />
(c) A complex of cobalt has the following composition by mass: Co, 21.98%; N, 31.35%; H, 6.72%; Cl, 39.75% (i) Calculate the empirical formula of this complex. [2] [1] (ii) The formula of this cobalt complex can be expressed in form [Co(L)m]x+(Cl)n Suggest the chemical formula of [Co(L)m]x+.empirical formula =SpecimenOCR 2014 H432/03<br />
BLANK PAGE SpecimenOCR 2014 H432/03 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/Specimen_QP_-_Paper_3_OCR_Chemistry_A_A-Level_2-0.jpg)
![H432/03 Question 2 (a)* Answer Marks 2, SO4 2 then Cl Please refer to the marking instructions on page 4 of this mark scheme for guidance on how to mark this question. Level 3 (56 marks) Develops a plan that allows identification of all six ions AND includes essential detail and equations for all test procedures and observations, with three anion tests in the correct sequence, CO3 AND includes cation test with essential detail and all equations There is a well-developed, detailed plan which is clear and logically structured. The plan is substantiated with relevant information, e.g. justification of the sequence of anion tests. There is a clear explanation of how the observations allow the ions to be identified. Level 2 (34 marks) Develops a plan that allows identification of at least three ions AND includes detail of at least three test procedures and observations, and three equations There is an appropriate plan presented with some structure. Parts of the fine detail, correct sequence, or reference to use of both samples may be missing. There is some attempt to explain how the observations allow the ions to be identified. Specimen Mark Scheme June 20XX Guidance Indicative scientific points may include: Use one sample for cation test, other sample for anion tests Details of tests Cation test add Aqueous sodium hydroxide Positive observationsfor Mn2+ : pink/buff precipitate for Fe2+ : green precipitate for NH4 of the tube turns blue + : litmus paper held over the opening + test (gentle) heating for NH4 Fine detail:Equations: Mn2+ + 2OHMn(OH)2 Fe2+ + 2OHFe(OH)2 NH4 Anion tests CO3add nitric acid; + + OHNH3 + H2O 2: positive observation: effervescence 2: SO4add aqueous barium nitrate; positive observation: white precipitate Cl:add silver nitrate solution; positive observation: white precipitate Fine detail for Cl: P M T<br />
H432/03 Question Mark Scheme June 20XX Answer Marks Guidance Level 1 (12 marks) Develops a plan that allows identification of at least two ions AND includes detail of at least two test procedures and observations, and one equation The plan is basic and communicated in an unstructured way. The response lacks fine detail and no reference to correct sequence of anion tests. There is little or no attempt to explain how the observations allow the ions to be identified. 0 marks No response or no response worthy of credit.subsequent addition of dilute ammonia solution positive observation: precipitate dissolves. Fine detail: correct sequence of all three anion testscarbonate test followed by sulfate test followed by halide test justification of sequence SpecimenALLOW splitting of solution over three boiling tubes/test tubes and performing each test on a different sample. 2BaSO4 2 + H+CO2 + H2O Equations: CO3 Ba2+ + SO4 Ag+ + ClAgCl Actual Kw = 2.381014 mol2 dm6 ALLOW ECF only if candidate uses a value between 2.0 and 2.61014 (mol2 dm6), i.e. from the approximately correct region of the graph ALLOW 6.8 (1DP) up to calculator value ALLOW ECF only if candidate has generated a value of [H+] by attempting to take a square root of a value between 2.0 and 3.01014 (b) Kw value from graph from 2.2 to 2.41014 (mol2 dm6) (cid:57) 10 4.2 Using 2.41014, [H+] = 14pH = log (1.55107) = 6.81 (using Kw = 2.41014) (cid:57) OR 1.55107 (cid:57) (c) (i) Co : N : H : Cl = 21.98 58.9 : 14.0 : 6.72 31.35 1.0 : 39.75 35.5 10 P M T<br />
H432/03 Question (ii) Mark Scheme June 20XX Answer Marks Guidance = 0.373 : 2.24 : 6.72 : 1.12 (cid:57) = 1 : 6 : 18 : 3 Formula = CoN6H18Cl3 (cid:57) [Co(NH3)6]3+ (cid:57) 12 Total Specimen 11 P M T<br />](https://learnah.org/wp-content/uploads/2018/08/Specimen_MS_-_Paper_3_OCR_Chemistry_A_A-Level_2-0.jpg)
