Metal-aqua ions and their acidityAQA A-Level Chemistry: Mind map

Exam questions on Metal-aqua ions and their acidity

1. A student measures the pH of two aqueous solutions, each of concentration 0.1 mol dm⁻³. Iron(II) nitrate solution has a pH of 3.7 and iron(III) nitrate solution has a pH of 1.9.
   Calculate how many times greater the concentration of H⁺ ions is in the iron(III) nitrate solution than in the iron(II) nitrate solution.2 marks
2. Aluminium chloride and copper(II) sulfate are each dissolved in water to make solutions of the same concentration. The aluminium chloride solution has a lower pH than the copper(II) sulfate solution. The ionic radius of Al³⁺ is 0.053 nm and that of Cu²⁺ is 0.073 nm.
   Use the data to explain why the aluminium chloride solution has a lower pH than the copper(II) sulfate solution.2 marks
3. Crevice corrosion occurs where stagnant water is trapped against steel. Iron dissolves to form iron(II) ions, which are oxidised by dissolved oxygen to iron(III) ions. Engineers find that the liquid trapped in the crevice becomes much more acidic than the surrounding water.
   Write an equation to show why a solution containing [Fe(H₂O)₆]³⁺ is acidic, and state the role of the aqua ion in this reaction.3 marks