Question

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).

Answer 1

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)

0.000755 0.000755
Hg(NO₃)₂ + Na₂S --> HgS + 2Na(NO₃)₂
1 1

Now you need to find volume of of 0.0783 mol/L sodium sulfide, that
includes 0.000755 mol of Na₂S

0.0783 mol mol per 1 L
0.000755 mol per V

V = 0.009642 L