Mercury compounds are poisonous, but mercury ions can be removed from a solution by precipitating insoluble mercury (II) sulfide, HgS (s). Determine the minimum volume of 0.0783 mol/L sodium sulfide, Na2S (aq), that is needed to precipitate all the mercury ions in 75.5 mL of 0.100 mol/L, Hg(NO3)2 (aq).
There is the next equation for this process:
Hg(NO3)2 + Na2S --> HgS + 2Na(NO3)2
For answering this question you need to find amount of Hg(NO3)2 in the solution:
0.100 mol per 1 L x mol per 0.0755 L
x = 0.000755 mol
0.000755 mol of Hg(NO3)2 can react with 0.000755 mol of Na2S (from equation)