Question

The ph at which a 0.01m al3+ solution is 99.99 precipitated is

Answer 1

We have to find the pH at which 0.01M Al³⁺ solution is 99.99 % precipitated. Here Ksp of Al(OH)₃ is 1 × 10^-18 at 25°C.

Solution : dissociation reaction of Al(OH)₃ is ...

Al(OH)₃ (aq) ⇒Al³⁺ (aq) + 3OH¯ (aq)

0. S. 3S

so, to find solubility product :

Ksp = [Al³⁺][3OH¯]³

Ksp = S(3S)³ = 27S⁴ = 10^-18

⇒S⁴ = (100/27) × 10¯²⁰

⇒S = 1.387 × 10¯⁵

Now 3[OH¯] = 3S = 3 × 1.387 × 10¯⁵

= 4.161 × 10¯⁵

So, pOH = -log(4.161 × 10¯⁵) = 4.38

Therefore the pH of Solution = 14 - pOH = 14 - 4.38 = 9.62