Question

Using the appropriate Ksp value from this table, calculate the pH of a saturated solution of Ca(OH)2.

Answer 1

Note: Ksp value is not provided. Following calculations are done by taking Ksp of Ca(OH)₂ as 6.5 X 10^-6 at 25^oC.

Ans:

The initial and equilibrium concentration of hydroxide ions can be calculated as follows:

Ca(OH)₂ = Ca²⁺   +    2OH^-

Initial conc.                         solid                       0              0             

Change in conc.                solid                       s              2s           

Equilibrium conc.              solid                       s              2s

Thus, expression for solubility product of Ca(OH)₂ can be given as follows:

Ksp = [Ca²⁺] [OH^-]²

Ksp = (s)(2s)²

Ksp = 4s³

Substitute value of Ksp = 6.5 X 10^-6

6.5 X 10^-6 = 4s³

s³ = (6.5 X 10^-6) / 4

s = 0.012

Thus, concentration of hydroxide ions can be calculated as follows:

[OH^-] = 2s

[OH^-] = 2 x 0.012

[OH^-] = 0.024

Therefore, pH of the solution can be calculated as follows:

pH = 14 – p[OH^-]

pH = 14-(-log[OH])

pH = 14 + log(0.024)

pH = 14 + (-1.6)

pH = 12.4

Thus, pH of the saturated solution of Ca(OH)₂ is 12.4