Acid-base titration

Question

1. An aqueous solution of Ba(OH)2 has a [H3O+] of 1 x 10^-11 M. What is [OH-]? What is the concentration of the solution?
2. An aqueous solution of iron (III) hydroxide has a pH of 11.59. What is the concentration of the solution?

Answer 1

1.) [OH^-] can be found by using K_w = [H₃O⁺][OH^-], given K_w = 1 x 10^-14 and [H₃O⁺] = 1 x 10^-11 M

[OH^-] = K_w / [H₃O⁺] = 1 x 10^-14 M² / 1 x 10^-11 M

[OH^-] = 1 x 10^-3 M

Because there are two moles of OH^- for every one mole barium, divide the concentration of OH^- by two to find the concentration of Ba(OH)₂, which is 5 x 10^-4 M.

2.) Finding [OH^-] first will be best. If the pH is 11.59, and since pH + pOH = 14, then

pOH = 14 - 11.59 = 2.41

[OH^-] = 10^-pOH = 10^-2.41 = 3.89 x 10^-3 M

Because there are three moles of [OH^-] for every one mole of iron, we can divide the concentration of [OH^-] by three to get the concentration of the iron (III) hydroxide solution, which is 1.297 x 10^-3 M.