OCR A Jun 2016 Paper 5 Q5

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5 A chemist carries out some experiments using nitrous acid, HNO2(aq). 10 HNO2 is a weak acid with a Ka value of 4.69104 mol dm3 at the temperature of the chemists experiments. (a) Write the expression for Ka for HNO2(aq). (b) Calculate the pH of 0.120 mol dm3 HNO2(aq). Give your answer to two decimal places. [1] (c) The chemist prepares 1dm3 of a buffer solution by mixing 200 cm3 of 0.200 mol dm3 HNO2 with 800 cm3 of 0.0625 mol dm3 sodium nitrite, NaNO2. pH =[2] (i) Calculate the pH of the buffer solution. Give your answer to two decimal places.OCR 2016 pH =[4]<br /> (ii) Explain how this buffer solution controls pH when:a small amount of HCl (aq) is added a small amount of NaOH(aq) is added. 11 In your answer, include the equation for the equilibrium in the buffer solution and explain how this equilibrium system controls the pH.[4]OCR 2016 Turn over<br /> (d) The dissociation of water is shown below. H2O(l) H+(aq) + OH(aq) 12 At 60 C, the ionic product of water, Kw , is 9.3111014 mol2 dm6. At 25 C, the ionic product of water, Kw, is 1.0001014 mol2 dm6. (i) Explain whether the dissociation of water is an exothermic or endothermic process.[1] (ii) Predict, using a calculation, whether a pH of 7 at 60 C is neutral, acidic or alkaline.[2] (iii) pKw , pKa and pH are logarithmic scales. Calculate pKw at 60 C. Give your answer to two decimal places. pKw =[1]OCR 2016<br /> (iv) 20.0 cm3 of 0.0270 mol dm3 NaOH is diluted with water and the solution made up to 100 cm3 at 60 C. 13 Calculate the pH of the diluted solution of NaOH at 60 C. Give your answer to two decimal places. pH =[3] [Total: 18]OCR 2016 Turn over<br />

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