Entropy is a measure of the disorder of a system. The enthalpy change of solution, ΔH°sol, of the Group 2 sulfates becomes more endothermic down the group. An aqueous solution of sodium hydroxide has a pH of 13.25 at 298 K. The Ka for ethanoic acid is 1.75 × 10⁻⁵ mol dm⁻³ at 298 K. Titration curves for two different acid-base reactions, M and N, are shown. [Figure showing two titration curves: reaction M (strong acid-strong base) and reaction N (weak acid-strong base)]

1. (a)State and explain the trend in the solubility of the Group 2 sulfates down the group. [3][3]
2. (b)(i)Write the expression for Kw, the ionic product of water. Kw = .............................................................. [1][1]
3. (b)(ii)The numerical value of Kw increases with increasing temperature. Place a tick (✔) in the appropriate column in each row to show the effect of increasing the temperature of water on the pH and on the ratio [H⁺]: [OH⁻]. | effect of increasing | decrease | stay the same | increase temperature of water | pH | | | ratio [H⁺]: [OH⁻] | | | [2][2]
4. (c)Calculate the concentration of this sodium hydroxide solution. concentration = ...................................... mol dm⁻³ [2][2]
5. (d)Buffer solutions are used to regulate the pH of a solution to keep its pH value within a narrow range. Write two equations to describe how hydrogencarbonate ions, HCO3⁻, and carbonic acid, H2CO3, control the pH of blood. [2][2]
6. (e)(i)When ethanoic acid is dissolved in water, an equilibrium mixture containing two acid-base pairs is formed. Write an equation for this equilibrium. In the boxes label each species acidic or basic to show its behaviour in this equilibrium. CH3CO₂H + [box] ........................ ⇌ ........................ + [box] ........................ [2][2]
7. (e)(ii)A buffer solution was prepared by adding 30.0 cm³ of 0.25 mol dm⁻³ ethanoic acid, an excess, to 20.0 cm³ of 0.15 mol dm⁻³ sodium hydroxide. Calculate the pH of the buffer solution formed at 298 K. Give your answer to one decimal place. pH = ...................................... [4][4]
8. (f)(i)Use the titration curve for reaction M to deduce the volume of acid added at the end-point for this titration. volume of acid added at the end-point = ...................................... cm³ [1][1]
9. (f)(ii)The table shows some acid-base indicators. name of indicator | pH range of colour change ------------------|-------------------------- malachite green | 0.2–1.8 bromocresol green | 3.8–5.4 bromothymol blue | 6.0–7.6 thymolphthalein | 9.3–10.6 Name a suitable indicator for each of the acid-base titrations M and N. Explain your answers. reaction M ..................................................... reaction N ..................................................... explanation ................................................................................................................................................................................................................................................................................................................ [2][2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.