Solubility trend of Group 2 sulfates Steps:

• Excess QCO3 added to H2SO4 reacts: QCO₃ + H₂SO₄ → QSO₄ + H₂O + CO₂.
• Excess insoluble QCO₃ remains as solid; filtration removes it.
• Filtrate holds QSO₄ only if soluble enough to avoid precipitation.
• Evaporation concentrates filtrate, crystallizing QSO₄ for final filtration to collect pure sample.

Why D is correct:

• Barium sulfate (insoluble, Ksp = 1.1 × 10⁻¹⁰) precipitates separately but process adapts via minimal dissolution; magnesium sulfate (highly soluble, >35 g/100 mL) fully enters filtrate—both yield pure QSO₄ without intermediate issues.

Why the others are wrong:

• A includes calcium, whose sulfate (sparingly soluble, 0.21 g/100 mL) partially precipitates, mixing with QCO₃ and contaminating separation.
• B includes calcium, same partial precipitation issue as above.
• C is calcium only, failing due to incomplete dissolution and mixed solids per solubility data.

Final answer: D