Will CoS precipitate in a solution that is 0.10 M in Co(NO3)2, 0....

Question

Will CoS precipitate in a solution that is 0.10 M in Co(NO3)2, 0.5 M in HCl, and 0.10 M in H2S? Will CoS precipitate if the pH of the solution is adjusted to pH 8 with an NH4+ - NH3 buffer? Ksp for CoS is 3.

Accepted Answer

Step 1: Identify the relevant chemical reaction for the precipitation of CoS. The reaction is: $$Co^{2+}$$ + $$S^{2-}$$ ightleftharpoons CoS(s).. Step 2: Calculate the concentration of sulfide ions $$(S^{2-}) in $$the solution. In acidic conditions, H2S is only partially dissociated, so use the dissociation equilibrium of H2S: H2S ightleftharpoons H^+ + HS^- ightleftharpoons 2H^+ + $$S^{2-}.. $$Step 3: Determine the concentration of $$S^{2-} in $$the acidic solution using the given concentration of HCl (0.5 M) and the initial concentration of H2S (0.10 M). Consider the effect of the strong acid (HCl) on the dissociation of H2S.. Step 4: Calculate the ion product (Q) for CoS using the concentrations of $$Co^{2+} (0.10 M) $$and the calculated $$S^{2-}$$ concentration. Compare Q to the Ksp of CoS (3 	imes $$10^{-21}) to $$determine if precipitation occurs.. Step 5: For the solution at pH 8, calculate the new concentration of $$S^{2-}$$ using the NH4^+ - NH3 buffer. Use the Henderson-Hasselbalch equation to find the ratio of NH3 to NH4^+ and adjust the $$S^{2-}$$ concentration accordingly. Recalculate Q and compare it to Ksp to determine if CoS will precipitate at pH 8.

Will CoS precipitate in a solution that is 0.10 M in Co(NO3)2, 0.5 M in HCl, and 0.10 M in H2S? Will CoS precipitate if the pH of the solution is adjusted to pH 8 with an NH4+ - NH3 buffer? Ksp for CoS is 3.

Key Concepts

Solubility Product Constant (Ksp)

Common Ion Effect

pH and Precipitation