CIE Jun 2015 v2 Paper 4 Q4

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(a) (i) What is meant by the term buffer solution?[2] (ii) Write equations to show how the hydrogencarbonate ion, HCO3 , controls the pH of blood.[2] (iii) A solution containing both Na2HPO4 and NaH2PO4 is commonly used as a buffer solution. The following equilibrium is present in the solution. H2PO4 (aq) HPO4 2(aq) + H+(aq) Ka = 6.2108 mol dm3 Calculate the pH of a buffer solution made by mixing 100 cm3 of 0.5 mol dm3 Na2HPO4 and 100 cm3 of 0.3 mol dm3 NaH2PO4. (b) Silver phosphate, Ag3PO4, is sparingly soluble in water. (i) Write an expression for the solubility product, Ksp, of Ag3PO4, and state its units. pH =[2] Ksp = units:[1] (ii) The numerical value of Ksp is 1.251020 at 298 K. Use this value to calculate [Ag+(aq)] in a saturated solution of Ag3PO4. [Ag+(aq)] =mol dm3 [3]UCLES 2015 9701/42/M/J/15<br /> (c) The half-equation for the redox reaction between phosphoric(III) acid and phosphoric(V) acid is shown. H3PO4(aq) + 2H+(aq) + 2e H3PO3(aq) + H2O(l) E o = 0.28 V Find suitable data from the Data Booklet to write an equation for the reaction between H3PO3 and Fe3+(aq) ions, and calculate the for the reaction. equation:=V [2] [Total: 12]UCLES 2015 9701/42/M/J/15 [Turn over<br />

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