If solid AgNO3 is slowly added to a solution that is .05 M in NaI and .12 M in NaCl, what is the concentration of I- when AgCl just begins the precipitate?

Ksp for AgCl= 1.8 x 10^-10

Ksp for AgI= 1.5 x 10^-16

Set up a ratio between Ksp AgCl/Ksp AgI and their respective expressions, and go from there. Post your work if you get stuck.

To determine the concentration of I- when AgCl just begins to precipitate, we can use the concept of the common ion effect. The common ion effect predicts that the solubility of a slightly soluble salt is reduced when a common ion is present in the solution.

Here's how you can approach this problem step by step:

1. Write the balanced equation for the precipitation of AgCl: AgNO3 + NaCl → AgCl + NaNO3

2. Determine the initial concentrations of the ions in the solution:
- [Na+] = 0.12 M (from NaCl)
- [I-] = 0.05 M (from NaI)
- [Ag+] = 0 M (before the addition of solid AgNO3)

3. As AgNO3 is added, it will dissociate into Ag+ and NO3-. The Ag+ ions from AgNO3 will react with Cl- ions to form AgCl and deplete the concentration of Cl- ions. Meanwhile, the Ag+ ions will react with I- ions to form AgI and deplete the concentration of I- ions.

4. Let's assume 'x' is the concentration of Ag+ that reacts with Cl- and I-. At the beginning, the concentrations of Cl- and I- are their initial concentrations.

5. The reaction between Ag+ and Cl- is Ag+ + Cl- → AgCl. Therefore, at equilibrium, the concentration of Cl- will be [Cl-] = 0.12 M - x.

6. The reaction between Ag+ and I- is Ag+ + I- → AgI. At equilibrium, the concentration of I- will be [I-] = 0.05 M - x.

7. Use the solubility product constant (Ksp) expression for AgCl to set up an equation:
Ksp = [Ag+][Cl-] = (x)(0.12 - x)

8. Use the solubility product constant (Ksp) expression for AgI to set up another equation:
Ksp = [Ag+][I-] = (x)(0.05 - x)

9. Plug the given Ksp values into the respective Ksp equations:
1.8 x 10^-10 = (x)(0.12 - x)
1.5 x 10^-16 = (x)(0.05 - x)

10. Solve the equations simultaneously to find the value of x, which represents the concentration of Ag+ ions that react with Cl- and I- ions.

11. Once you find the value of x, substitute it back into the expression [I-] = 0.05 M - x to calculate the concentration of I- when AgCl just begins to precipitate.

Following these steps will allow you to calculate the concentration of I- when AgCl just begins to precipitate.