Question

A solution contains 1.0 × 10^–5 mol of KBr and 0.10 mol of KCl per liter. AgNO3 is gradually added to this solution. Which forms first, solid AgBr or solid AgCl?

a) Solid AgBr

b) Solid AgCl

c) Both form simultaneously

d) It cannot be determined

Answer 1

AgCl will form first when AgNO3 is added to the solution as it requires a lower silver ion concentration to begin precipitating compared to AgBr.

Step-by-step explanation:

When AgNO3 is added to a solution containing both KBr and KCl, we need to calculate the silver ion concentration [Ag+] at which each corresponding silver halide (AgBr and AgCl) begins to precipitate. This is determined by their solubility products (Ksp) and the concentrations of Br- and Cl-.

The Ksp for AgBr is lower than that of AgCl, which means AgBr is less soluble and would typically precipitate first. However, the amount of each ion in solution affects the reaction quotient Q, and the one that exceeds its Ksp first will precipitate.

For AgBr, which starts to precipitate when [Ag+] is 5.0 × 10-9 M, while AgCl starts to precipitate when [Ag+] equals 1.6 × 10-9 M. Since the concentration of [Ag+] needed to begin precipitation of AgCl is lower, AgCl will form first.