Question

1. Determine the [OH–] in 3 × 10–5 M solution of HCl.
2. Determine the pH of 2.0 × 10–2 M Sr(OH)2 solution assuming complete
dissociation.

Answer 1

1. HCl is a strong acid and completely dissociates in water according to the equation HCl → H+ + Cl-. Since there are no hydroxide ions present in the solution, the [OH-] is 0.

2. Sr(OH)2 is a strong base and completely dissociates in water according to the equation Sr(OH)2 → Sr2+ + 2OH-. The concentration of hydroxide ions in 2.0 × 10-2 M Sr(OH)2 solution is twice the concentration of the dissolved compound (since for every 1 mole of Sr(OH)2 there are 2 moles of OH- ions).

[OH-] = 2.0 × 10-2 M × 2 = 4.0 × 10-2 M

pH + pOH = 14
pOH = -log[OH-] = -log(4.0 × 10-2) = 1.40

pH = 14 - pOH = 14 - 1.40 = 12.60