Question

What solution can you add to of the mixture of sr2+(aq) and hg22+(aq) to precipitate one cation while keeping the other cation in solution?

Answer 1

We can use any solution containing chloride ions as NaCl solution or even dilute HCl due to mercurous Hg₂²⁺ is from group I cations which precipitated as chloride while Sr²⁺ is from group V 
Net ionic equation:
Hg₂²⁺ + 2 Cl⁻  → Hg₂Cl₂ (white precipitate)
while SrCl₂ is soluble salt