Question

a)

What is the solubility of magnesium hydroxide in a solution buffered at pH 9.15?

s = _____g/L

b)

What is the molar solubility of Al(OH)₃ in a solution containing 1.34E-2 M NaOH? K_sp (Al(OH)₃) = 4.6E-33.

= _____M

c)

What must be the concentration of chromate ion in order to precipitate strontium chromate, SrCrO₄, from a solution that is 3.1E-3M Sr²⁺? The K_sp for strontium chromate is 3.5E-5.

= _____M

Answer 1

a) pH = 9.15

   pH + pOH = 14

pOH = 4.85

[OH-] = 10^-pOH

[OH-] = 1.41 x 10^-5

Mg(OH)2--------------------------> Mg+2 + 2OH-

                                                S            1.41 x 10^-5

Ksp = [Mg+2][OH-]^2

5.6x10-^12 = S (1.41 x 10^-5)^2

S = 0.0281

solubility = 0.281 mol/ lit

Mg(OH)2 molar mass = 58 g /mol

solubility = 0.281 / 58

              = 4.84 x 10^-3 g / Lit

b)

Al(OH)3 ------------------------------> Al+3   + 3OH-

                                                    S           1.34 x10^-2

Ksp = (S) (1.34 x10^-2)^3

4.6 x 10^-33 = (S) (1.34 x10^-2)^3

S = 1.9 x 10^-27

molar solubility = 1.9 x 10^-27 M

C)

SrCrO4--------------------------> Sr+2      + CrO4-2

                                             3.1 x10^-3     S

Ksp = [sr+2][CrO4-2]

3.5 x10^-5 = 3.1 x 10^-3 [CrO4-2]

[CrO4-2 ] = 0.0113 M

in order to precipitate chromate ion concentration should be more then 0.0113 M   ([CrO4-2 ] > 0.0113 M)