Solubilization reactions:
Co(NO3)2(aq) → Co2+(aq) + 2 NO3–(aq)
Acid/base reactions:
H2S(aq) + H2O(l) → ← H3O+(aq) + HS–(aq)
HS–(aq) + H2O(l) → ← H3O+(aq) + S2–(aq)
Precipitation reaction:
Co2+(aq) + S2–(aq) → CoS(s)
Net reaction:
Co(NO3)2(aq) + H2S(aq) + 2 H2O(l) → CoS(s) + 2 H3O+(aq) + 2 NO3–(aq)