Calculate the solubility

Question

Calculate the solubility (in mol/L) of Co(OH)3 (Ksp = 3 ✕ 10−43) in each of the following. (Assume that all solutions are at 25°C. Enter unrounded values.)

(a) water (assume pH is 7.0 and constant) = _____

(b) a solution buffered at pH = 5.5 =________

(c) a solution buffered at pH = 11.3 = _______

Answer 1

Co(OH)₃-----> Co³⁺ + 3OH^- Ksp = 3 ✕ 10⁻⁴³ Ksp = [Co³⁺][OH^-]³ [Co(OH)₃]= [Co³⁺] = x

(a) pH = 7.0 [OH^-] = 1x10^-7 3 ✕ 10⁻⁴³= x (1x10^-7)³ x = [Co(OH)₃] = 3.0 x 10^-22 M

water (assume pH is 7.0 and constant) = 3.0 x 10^-22 M

(b) pH = 5.5 pOH=14-5.5 = 8.5 [OH^-] = 10^-8.5 = 3.16 x 10^-9 M 3 ✕ 10⁻⁴³= x (3.16 x 10^-9 )³

x = [Co(OH)₃] = 9.49 x 10^-18 M a solution buffered at pH = 5.5 = 9.49 x 10^-18 M

(c) pH = 11.3 pOH=14- 11.3 =2.7 [OH^-] = 10^-2.7 = 0.002 M 3 ✕ 10⁻⁴³= x (0.002 )³

x = [Co(OH)₃] = 3.75 x 10^-35 M a solution buffered at pH = 11.3 = 3.75 x 10^-35 M