2. Magnesium hydroxide has a solubility of 9.24 × 10 –4grams per 100 mL H 2 O when
   measured at 25°C.

(a) Write a balanced equation representing magnesium hydroxide in equilibrium in a

water solution.

(b) Write an equilibrium expression for magnesium hydroxide in water.

(c) Calculate the value ofKspat 25°C for magnesium hydroxide.

(d) Calculate the value of pH and pOH for a saturated solution of magnesium hydroxide

at 25°C.

(e) Show by the use of calculations whether a precipitate would form if one were to add

75.0 mL of a 4.00 × 10 –4M aqueous solution of magnesium chloride to 75.0 mL of a

4.00 × 10 –4M aqueous solution of potassium hydroxide.

Answer

2. Given: Mg(OH) 2 solubility = 9.24 × 10 –4g/100 mL H 2 O at 25°C.
   (a) Restatement: Balanced equation in equilibrium.
   Mg(OH) 2 (s) ↔Mg2+(aq) + 2OH–(aq)
   (b) Restatement: Equilibrium expression.
   Ksp= [Mg2+][OH–]^2
   (c) Restatement: Value of Ksp.
   MW Mg(OH) 2 = 58.33 g/mole

.

.()

mL H O ()

gMg OH

gMg OH

mole Mg OH

liter H O

mL H O

58 33

1

1

1000

100

924 10 2

2 2

(^42)
2
2

- ^h

= 1.58 × 10 –4M Mg(OH) 2
= 1.58 × 10 –4M Mg2+
= 2(1.58 × 10 –4) = 3.16 × 10 –4M OH–
Ksp= [Mg2+][OH–]^2 = (1.58 × 10 –4)(3.16 × 10 –4)^2
= 1.58 × 10 –11
(d) Restatement: pH and pOH.
pOH = −log[OH–]
= −log (3.16 × 10 –4) = 3.5
pH = 14.0 −pOH = 10.5
(e) Given: Add 75.0 mL of 4.00 × 10 –4M MgCl 2 to 75.0 mL of 4.00 × 10 –4M KOH.
Restatement: Would a precipitate form?
MgCl 2 →Mg2+(aq) + 2 Cl–(aq)
KOH →K+(aq) + OH–(aq)
Equilibrium