Question

Chromium(III) hydroxide is insoluble in water, but highly soluble at low pH and high pH.

a. Calculate the molar solubility of Cr(OH)3 in water. You may ignore the hydroxide produced from

the auto-ionization of water.

b. Why is Cr(OH)3 soluble in solutions having low pH? The number of points awarded for this

question is proportional to the detail you give in your answer.

c. Cr(OH)3 is soluble at high pH due to the formation of the tetrahydroxochromate(III) ion, Cr(OH)4–.

Calculate the pH at which Cr(OH)3 will have a solubility of 0.10 M under these conditions.

Answer 1

Cr(OH)3 -------->Cr+3 +3OH-

KSp = [Cr+3] [OH-]³

let x= solubility of Cr(OH)3

KSp = (3x)³*x = 6*10^-31

27x⁴= 6*10-31

x= 1.22*10^-8 M

2. Cr(OH)3 ------> Cr+3 + 3OH- KSP= 6*10^-31

Cr+3 + 4OH- ------->Cr(OH)4- Kf= 8*10²⁹

addition gives Cr(OH)3 + OH- ------->Cr(OH)4- K=   6*10^-31* 8*10²⁹ =0.48

   Cr(OH)3    OH- Cr(OH)-4

initilal        0.1M                   0                                0

K= [Cr(OH)4- /[OH-]

0.48= 0.1/ [OH-]

[OH-] = 0.1/0.48= 0.21

pOH= 0.677

pH= 14-0.677= 13.323