Answer to Question #101335 in Chemistry for Julia

Question

calculate the pH of a saturated solution of magnesium hydroxide

Answer 1

Magnesium hydroxide dissociates according to the following equation:

Mg(OH)₂ ⇌ Mg²⁺ + 2OH^-

Solubility product (Ksp) of magnesium hydroxide at standard conditions equals:

K_sp = 5.61 10¯¹²

As a result:

K_sp = [Mg²⁺] × [OH^-]²

From here:

K_sp = (s) × (2s)² = 4s³

s = (4K_sp)^1/3 = 1.12 × 10^-4 M

As a result, concentration of hydroxide ions equals:

[OH¯] = 2 × 1.12 × 10^-4 M = 2.24 × 10^-4 M

From here:

pOH = - log (2.24 × 10^-4) = 3.650

pH = 14 - 3.650 = 10.350

Answer: 10.350