Answer: B (add excess aqueous barium nitrate, dry and weigh the precipitate). This is the standard gravimetric test for sulfate ions. Barium ions react with sulfate ions to give a white precipitate of barium sulfate, BaSO₄, which is highly insoluble in water: \(\text{Ba}^{2+} + \text{SO}_4^{2-} \to \text{BaSO}_4\). The mass of dried precipitate gives the moles of sulfate directly.
Crucially, barium chloride, BaCl₂, is fully soluble, so the chloride ions also present in sea water are not precipitated and do not interfere.
Option A uses lead(II) nitrate, but lead ions also form sparingly soluble lead(II) chloride, PbCl₂, so the precipitate would be a mixture and overstate the sulfate. Evaporating the water (C) leaves all dissolved solids, not just sulfate, and measuring electrical conductivity (D) reflects every ion in solution, not one specific ion.
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