Question

A bottle in lab is labeled [CoCl2.6H2O] = 0.728 M in 4.983 M HCl. If you determine [CoCl42-] to be 0.136 M at a particular temperature using absorbance measurements, what must be the value of [Co(H2O)62+] in the solution at that temperature? Express your response to three digits after the decimal.

Answer 1

Ans. Given

            [CoCl₂.6H₂O] = 0.728 M in 4.983 M HCl.

When CoCl₂.6H₂O is dissolved in aqueous HCl, the following equilibrium is established-

            CoCl₄²⁺ + 6H₂O <-----------> Co(H₂O)₆²⁺ + 4 Cl^-

All the CoCl₂.6H₂O is present only in form of CoCl₄²⁺ and Co(H₂O)₆²⁺. Adding more of H⁺ shifts equilibrium to the right, Co(H₂O)₆²⁺, blue-colored complex. Adding more water shifts equilibrium to the left, CoCl₄²⁺, red (pink)-colored complex. The relative abundance of the two chemical species follows Le Chetalier’s principle.

However, at any moment, irrespective of the position of equilibrium, the sum of concentration of CoCl₄²⁺ and Co(H₂O)₆²⁺ is equal to the initial concentration of CoCl₂.6H₂O. That is,

            [CoCl₂.6H₂O] = [CoCl₄²⁺] + [Co(H₂O)₆²⁺]

Putting the given concentration in above equation-

            0.728M = 0.136 M + [Co(H₂O)₆²⁺]

            Or, [Co(H₂O)₆²⁺] = 0.728M - 0.136 M = 0.592 M

Thus, [Co(H₂O)₆²⁺] = 0.592 M in the given solution.