Cobalt ion exists as [CoCl_4]^2+ in aqueous solutions. The following equilibrium

Question

Cobalt ion exists as [CoCl_4]^2+ in aqueous solutions. The following equilibrium reaction represents the two types of complexes of cobalt with two different colors. [CoCl_4]^2-(aq, blue) + 6H_2O(l) [Co(H_2O)_6]^2+ (aq, pink) + 4 cl^-(aq) a. What change in color do you expect when you add 10 drops of 12 M HCl to an aqueous solution of cobalt salt? Explain your answer in terms of LeChatlier principle. b. To the above mixture, if you add distilled water drop-wise what change you expect? Explain your answer in terms of LeChatlier principle. Answer the following questions. a) A certain colorless solution has a pH of 10. i) If we add phenolphthalein indicator to this solution, what will be the color of the resulting solution? Explain. ii) Now, suppose we add an acid drop-wise to the solution from solution (i) what will be observed as we add the acid? Explain. b) The pH of a certain solution is 8.0. What color would the indicators methyl violet, methy orange, and phenolphthalein be in this solution? Methyl violet _____ Methyl orange _____ Phenolphthalein _____ c) Methyl violet is violet in a given solution. Methyl orange is red in the same solution. What will be the approximate pH of the solution? Briefly explain.

Explanation / Answer

Q3.

a) We expect the blue colour to intensify as addition of HCl will increase the concentration of [Cl-] ion thereby leading to backward shift of equilibrium as per Le-Chatlier's principle.

b) We expect the pink colour to intensify as addition of H2O leads to increase in volume of the system in equilibrium and increase in volume leads to shifting of equilibrium towards the side with more no. of moles of gaseous and aqueous substance.

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