Chemistry

Problem

A saturated solution of Mg(OH)

_2

is at equilibrium at 25 °C. Solid Mg(OH)

_2

remains in contact with the solution. A small amount of solid NaOH is added, raising the pH from 10 to 12. Which of the following best describes the resulting change in the concentration of Mg

^{2+}

ions in the solution, and the reasoning behind it? a) [Mg

^{2+}

] remains unchanged because adding NaOH does not alter the value of

K_{sp}

for Mg(OH)

_2

. b) [Mg

^{2+}

] increases because more Mg(OH)

_2

must dissolve so that the product

[\text{Mg}^{2+}][\text{OH}^-]^2

equals the fixed

K_{sp}

. c) [Mg

^{2+}

] decreases only slightly because OH

^-

is a spectator ion that does not participate in the precipitation reaction. d) [Mg

^{2+}

] decreases because the added common ion OH

^-

shifts the dissolution equilibrium toward solid Mg(OH)

_2

, precipitating Mg(OH)

_2

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