Question

What is the pH of a saturated solution of Ni(OH)₂ ?

pH=

What is the solubility in grams of Ni(OH)₂ /100 mL of solution?

Solubility = g/100 mL

Answer 1

take the Ksp value of Ni(OH)2 from the standard table

Ksp of Ni(OH)2 = 5.5 x 10^-16

Ni(OH)2 <----> Ni2+ + 2OH-

Ksp = [Ni2+] [OH-]²

lets s is the molar solubility

Ksp = [s][2s]² = 4s³

5.5 x 10^-16 = 4s³

s³ = 5.5 x 10^-16 / 4 = 1.375 x 10^-16

s = qubic root of 1.375 x 10^-16 = 5.16 x 10^-6 mol /L

from the Ni(OH)2 2 OH- are comming

so OH- = 2 x 5.16 x 10^-6 = 1.0 x 10^-5

pOH = -log (OH-) = -log (1.0 x 10^-5 ) = 5

pH = 14-pOH = 14 - 5 = 9

Part B

Ksp of Ni(OH)2 = 5.5 x 10^-16

Ni(OH)2 <----> Ni2+ + 2OH-

Ksp = [Ni2+] [OH-]²

lets s is the molar solubility

Ksp = [s][2s]² = 4s³

5.5 x 10^-16 = 4s³

s³ = 5.5 x 10^-16 / 4 = 1.375 x 10^-16

s = qubic root of 1.375 x 10^-16 = 5.16 x 10^-6 mol /L

to convert mol/L to gram /L

we have to multply with molecular weight of Ni(OH)2

5.16 x 10^-6 mol /L(92.70 g /mol)

= 4.78 x 10^-4 g /L

but we need g / 100 mL

4.78 x 10^-4 g are there in 1L

divide with the above value with 100

4.78 x 10^-4 / 100

= 4.78 x 10^-6 g /100 mL