Some 1.0 mol dm⁻³ solutions were mixed using equal volumes of each solution - AQA - A-Level Chemistry - Question 31 - 2018 - Paper 3

To determine which pair of solutions will yield the greatest mass of solid when mixed, we need to consider the solubility product (Ksp) of the possible precipitation reactions.

1. Identify the possible reactions:

   • Mixing Ba(OH)₂ with MgCl₂:

     • Ba(OH)₂ dissociates to release Ba²⁺ and 2 OH⁻ ions.
     • MgCl₂ dissociates to release Mg²⁺ and 2 Cl⁻ ions.
     • This may lead to a precipitate of Ba(OH)₂, but both ions may remain soluble.

   • Mixing Ba(OH)₂ with MgSO₄:

     • The reaction may form BaSO₄, which is insoluble.

   • Mixing Ba(OH)₂ with NaCl:

     • NaCl doesn't lead to a solid precipitate with Ba(OH)₂

   • Mixing Ba(OH)₂ with Na₂SO₄:

     • Similar to the previous case, this could also lead to a precipitate of BaSO₄.

2. Determine solubility:

   • The formation of a solid from Ba(OH)₂ and either MgSO₄ or Na₂SO₄ is most likely, as both reactions could produce BaSO₄, which is known to be insoluble.

3. Conclusion:

   • The pair of solutions that would give the greatest mass of solid is therefore D. Ba(OH)₂ and Na₂SO₄, as it leads to the formation of BaSO₄.