The concentration of OH⁻ ions at 298K in a saturated solution of magnesium hydroxide, a sparingly soluble electrolyte, is 4 x 10⁻⁴ M. What is the solubility product of the salt at 298K?
a) 1.6 x 10⁻⁸
b) 3.2 x 10⁻⁸
c) 6.4 x 10⁻⁸
d) 8.0 x 10⁻⁸