CIE Nov 2016 v2 Paper 4 Q5

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The phosphate buffer system operates in biological cells. The buffer contains dihydrogen phosphate, H2PO4 , which acts as a weak acid. PMT H2PO4+ H2O HPO4 2 + H3O+ (a) Write an expression for the Ka of H2PO4 . Ka = [1] (b) (i) Explain what is meant by the term buffer solution.[2] (ii) Write two equations to show how a solution containing a mixture of H2PO4 acts as a buffer.and HPO4 2[2] (c) The pH in many living cells is 7.40. H2PO4+ H2O HPO4 2 + H3O+ Ka = 6.31108 mol dm3 Calculate the value of [HPO4 2] / [H2PO4 ] needed to give a pH of 7.40 in the cells. [HPO4 2] / [H2PO4 ] =[3] (d) (i) The H2PO4ion can also act as a base. Write an equation to show H2PO4acting as a base.[1] (ii) The HPO4 2 ion can also act as an acid. Write an equation to show HPO4 2 acting as an acid.[1]UCLES 2016 9701/42/O/N/16 [Total: 10] [Turn over<br />

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