Q1(b). Selective sulfate precipitation

Answer in brief

BaSO₄ precipitates first for equal or comparable metal concentrations; the rigorous comparison uses Kₛₚ/[M²⁺].

Key points

• Precipitation begins when [SO₄²⁻]_(crit)=Kₛₚ/[M²⁺] under a concentration-based approximation.
• For equal metal concentrations, barium sulfate reaches saturation at a much lower sulfate concentration.
• More generally, BaSO₄ precipitates first if [Ba²⁺]/[Sr²⁺]>4.2×10⁻¹¹/(2.8×10⁻⁷)=1.5×10⁻⁴.
• The metal concentrations are omitted, so an unconditional order cannot be inferred from solubility products alone. The customary intended answer assumes comparable concentrations.