12201 17 The dissociation of water is an endothermic process. At 60 °C the ionic product of water, Kw, has a value of 9.3 × 10−14 mol2 dm−6. 2H2O(l)   ⇌    H3O+(aq)    +    OH−(aq) (a) (i) Write the expression for Kw. [1] (ii) Explain why the expression for Kw does not include the concentration of water. [1] (b) (i) Calculate the pH of water at 60 °C. Give your answer to two decimal places. [2] (ii) Explain whether water at 60 °C is neutral, acidic or alkaline. [2] 12201 [Turn over (iii) Calculate the pH of a solution containing 256.5 g of barium hydroxide dissolved in 500 cm3 of water at 60 °C. Give your answer to two decimal places. [4] (c) (i) An acidic buffer is formed when 10.0 cm3 of 0.12 mol dm−3 aqueous sodium hydroxide is added to 25.0 cm3 of 0.18 mol dm−3 ethanoic acid. Calculate the pH of this buffer. The pKa of ethanoic acid is 4.76. Give your answer to two decimal places. [4] (ii) Explain, using an equation, the effect of adding small amounts of acid to the buffer. [2]