What is the pH?

Question

What is the pH of a 0.450 M solution of Ca(NO₂)₂ (Ka of HNO₂ is 4.5 × 10⁻⁴)?

Answer 1

When given a salt like this, you need to look at the hydrolysis. Since Ca(NO₂)₂ is the salt of a strong base, Ca(OH)₂ and a weak acid, HNO₂, the pH of the salt should be basic (alkaline, pH >7).

The hydrolysis of NO2- is as follows:

NO₂^- + H2O ==> HNO₂ + OH^-. We now need the Kb for NO₂^- (it is acting as a base here)

KaKb = 1x10^-14 and Kb =1x10^-14 / 4.5x10^-4 = 2.22x10^-11

Kb = 2.22x10^-11 = [HNO₂][OH^-] / [NO₂^-]

[HNO₂] = [OH^-] = x and [NO₂^-] = 0.450 x 2 = 0.900

2.22x10^-11 = (x)(x) / 0.0.900-x (ignore x in denominator since Kb is so small)

2.22x10^-11 = x² / 0.450

x² = 1.98x10^-11

x = 4.45x10^-6 M = [OH^-]

pOH = -log 4.45x10^-6

pOH = 5.35

pH = 14 - 5.35

pH = 8.65 (basic as predicted)