Sulfuric acid, H2SO4, reacts with sodium hydroxide, NaOH, as shown. H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l) A sample of 25.0 cm³ of 0.0500 mol/dm³ H2SO4 is placed in a beaker. NaOH(aq) is added slowly, from a burette, to the H2SO4 in the beaker. A pH probe is used to measure the pH of the solution in the beaker until a total of 40.0cm³ of NaOH(aq) is added. The graph shows how the pH of the solution in the beaker changes. [Figure pH titration curve showing pH on y-axis from 0 to 14, and volume of NaOH(aq) added/cm³ on x-axis from 0 to 40. The curve starts at pH 1, gradually rises, then rapidly rises around 20 cm³ of NaOH added, levelling off at pH 13.]

1. (a)Explain, in terms of the ions present, why the pH of the solution in the beaker changes from 1.0 to 13.0.[2]
2. (b)Use the graph to state the volume of NaOH(aq) that just neutralises all of the H2SO4· volume of NaOH(aq) ................................................................ cm³ [1][1]
3. (c)Use your answer to (b) to calculate the concentration, in mol/dm³, of the NaOH(aq). concentration of NaOH(aq) ................................................ mol/dm³ [3][3]

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