Question

A solution of 0.075 M CoBr₂ is saturated with H₂S ([H₂S] = 0.10 M). What is the minimum pH at which CoS begins to precipitate?

a) 2
b) 4
c) 6
d) 8

Answer 1

To determine the minimum pH at which CoS begins to precipitate, we need to consider the equilibrium between H₂S and CoS. We can calculate the concentration of S²⁻ ions and find the corresponding pH at which CoS begins to precipitate.

Step-by-step explanation:

To determine the minimum pH at which CoS begins to precipitate, we need to consider the equilibrium between H₂S and CoS. The balanced equation for the reaction is:

H₂S(aq) + CoBr₂(aq) ⇌ CoS(s) + 2HBr(aq)

The solubility product expression for CoS is given as:

Ksp = [Co²⁺][S²⁻]

At the minimum pH, the concentration of S²⁻ ions will be equal to the solubility product of CoS. So, we need to calculate the concentration of S²⁻ ions at a given pH.

Using the given equilibrium equation and the given concentrations of H₂S and CoBr₂, we can calculate the concentration of S²⁻ ions and find the corresponding pH at which CoS begins to precipitate.