A 0.5 M solution of hydrogen sulfide (H2S) contains 0.15 M nickel chloride NiCl2, and 0.35 M mercury nitrate Hg(NO3)2. The pH of this solution is made very acidic through the use of HCl so that all ions are in solution (see section 18.4). As the pH is made more basic by adding aqueous ammonia, what pH is required to precipitate the maximum amount of HgS but none of the NiS? Remember, for H2S use the values listed in Table 17.1 (not the footnote).

a. pH = 1.2

b. pH = 1.5

c. pH = 1.75

d. pH = 2.3

e. pH = 3.8