Silver (I) can form a precipitate and/or a complex ion when it is mixed with sod

Question

Silver (I) can form a precipitate and/or a complex ion when it is mixed with sodium chloride, depending on the concentration of chloride ion present in the solution.

Write the reaction for the formation of the complex ion and the reaction for the formation of the precipitate .

What concentration of Cl- is required to form a precipitate of AgCl (s) (2 pts)? ([Ag+] = 0.1M; [NaCl] =1M). Your answer should be listed as a molar concentration.

What concentration of Cl- is required to form a complex ion of [Ag(Cl) 2]-(aq) ? Your answer should be listed as a molar concentration.

Explanation / Answer

Reaction for formation of precipitate: Ag+ (aq) + Cl-(aq) = AgCl (s)

Reaction for formation of complex ion= AgCl (s) + Cl- (aq) = [AgCl2]-

To solve the next two parts of the question we will need the following values from literature:

Ksp (AgCl) = 1.77 X 10-10

Kf (AgCl2-) = 1.1 X 105

1. Concentration of Cl- to form AgCl precipitate:

Let it be x.

Ksp = [Ag+][Cl-]

1.77 X 10-10 = (0.1)(x)

x= 1.77 X 10-9 M

2. Concentration of Cl- required to form complex ion:

The formation of complex ion can be represented as an addition of the reaction equations for solubility of AgCl and formation of AgCl2- from its constitutent ions as follows:

AgCl = Ag+ + Cl- Ksp= 1.77 X 10-10 Equation 1

Ag+ + 2Cl- = AgCl2- Kf= 1.1 X 105 Equation 2

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AgCl + Cl- = AgCl2- K= Ksp. Kf Equation 3 (Equation 1 + Equation 2)

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Equilibrium constant K for the above final reaction = Ksp. Kf

K= 1.77 X 10-10 . 1.1 X 105 = 1.9 X 10-5

Also, K= [AgCl2-]/ [Cl-][AgCl (s)] Equation 4   

Now, if the solubility of AgCl in Cl- (equation 3) is y, then at equilibrium [AgCl2-]= y

and [Cl-] = 1- y (Since Cl- is being used from the 1M NaCl present in solution)

[AgCl (s)] =1

Substituting it in Equation 4,

1.9 X 10-5 = y/ (1-y)

Next, 1.9 X 10-5 - 1.9 X 10-5y = y

y + 1.9 X 10-5y = 1.9 X 10-5

y = 1.9 X 10-5 (y >>> 1.9 X 10-5y)

This will be the minimum concentration of Cl- for complex formation. Any concentration above this will lead to complexation.

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