OCR A Jun 2011 Paper 5 Q4

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4 Chemists and biochemists use pKa values to compare the strengths of different acids. pKa is a more convenient way of comparing acid strengths than Ka values. pKa values of several naturally occurring BrnstedLowry acids are shown in Table 4.1 below. common name and source systematic name structural formula benzoic acid (from bark resin) acetic acid (from vinegar) benzenecarboxylic acid C6H5COOH ethanoic acid CH3COOH pyruvic acid (formed during metabolism) 2-oxopropanoic acid CH3COCOOH lactic acid (from milk) 2-hydroxypropanoic acid CH3CHOHCOOH pKa (at 25 C) 4.19 4.76 2.39 3.86 (a) (i) What is meant by the term BrnstedLowry acid ? Table 4.1[1] (ii) What is meant by the strength of an acid? In your answer, include an equation for one of the acids in Table 4.1.[2] (iii) Place the four acids in Table 4.1 in order of increasing strength. weakest acidstrongest acid[1] (iv) Aqueous benzoic acid was mixed with aqueous lactic acid. An equilibrium mixture was formed containing conjugate acidbase pairs. Complete the equilibrium below to show the components in the equilibrium mixture. C6H5COOH + CH3CHOHCOOH+[1]OCR 2011<br /> (b) Aqueous pyruvic acid was reacted with an aqueous solution of calcium hydroxide. (i) Write an equation for this reaction.[1] (ii) Write an ionic equation for this reaction.[1] (c) The pH of an acid solution can be calculated from its pKa value. Calculate the pH of a 0.0150 mol dm3 solution of pyruvic acid at 25 C. Show all your working. Give the pH to two decimal places. pH =[4]OCR 2011 Turn over<br /> (d) Oxalic acid (ethanedioic acid), C2H2O4, is present in the leaves of rhubarb plants. Oxalic acid has two dissociations with pKa = 1.23 and pKa = 4.19. 10 (i) Draw the structure of oxalic acid. (ii) Predict the equations that give rise to each dissociation. pKa = 1.23 pKa = 4.19 [1] [2] (e) The magic tang in many sweets is obtained by use of acid buffers. A sweet manufacturer carried out tasting tests with consumers and identified the acid taste that gives the magic tang to a sweet. The manufacturer was convinced that the magic tang would give the company a competitive edge and he asked the companys chemists to identify the chemicals needed to generate the required taste. The chemists findings would be a key factor in the success of the sweets. The team of chemists identified that a pH of 3.55 was required and they worked to develop a buffer at this pH. The chemists decided to use one of the acids in Table 4.1 (page 8) and a salt of the acid to prepare this buffer.Deduce the chemicals required by the chemists to prepare this buffer. Calculate the relative concentrations of the acid and its salt needed by the chemist to make this buffer. Comment on the validity of the prediction that the pH of the sweet would give the sweets the magic tang.OCR 2011<br /> 11[6]OCR 2011 [Total: 20] Turn over<br />

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