Question

1) Calculate pH of saturated solution of silver nitrite (AgNO2) in water if its solubility

product is Ksp= 6.1*10^-4 and the pKa of nitrous acid is 3.4.

2) Dibasic calcium phosphate (CaHPO4) has limited solubility in water with solubility product of Ksp= 7.23*10^-7. Taking into account ionic interactions, what will be the concentration of Ca2+ ions in the solution saturated with CaHPO4 after 2.5*10^-3 m magnesium chloride (MgCl2) is dissolved in the same solution.

Answer 1

1.

AgNO2 (s)      =   Ag⁺    +       NO₂^-             K = Ksp = [Ag+][NO₂^-]

NO₂^-   + H2O = HNO2 +   HO^-                 K = Kb = [HNO2][HO^- ]/[ NO₂^-]

…………………………………………………………………………..       

AgNO2 (s) + H2O =   Ag⁺    +   HNO2 +   HO^-       K = Ksp.Kb

Observe that:

[HNO2] = [HO^-]

And [Ag+] = [NO₂^-] = (Ksp)^1/2

Ksp.Kb = [Ag+][HNO2][HO-]

1x10^-14Ksp/Ka = (Ksp)^1/2 x [HO-]²

1x10^-14Ksp^1/2/Ka = [HO-]²

[HO-]² = 1x10^-14x(6.1x10^-4)^1/2/10^-3.4

            = 2.45x10^-16 / 4.0x10^-4

            = 61.25x10^-14

[HO-] = 7.8x10^-7 M

[H+] = 1x10^-14/7.8x10^-7 = 1.3x10^-8 M

pH = -log[H+] = 7.9

Reload q2 as separate post.