Question

An amphoteric salt is one that contains an anion that can act as either an acid or a base in water. Baking soda, NaHCO3, is an example. By combining the ionization and hydrolysis reactions of the anion, you get the principle reaction that occurs when this salt is dissolved in water.

2HCO3-(aq) CO32-(aq) + H2CO3(aq)
The pH for such, a solution is given by
pH = pKa1 + pKa2/2
where Ka1 and Ka2 are the ionization constants of the acid (in this case, H2CO3). Note that the pH of the solution is independent of the salt concentration. Calculate the pH of a NaHCO3 solution.

Answer 1

pH = 8.34

Step-by-step explanation:

The equilbriums of the amphoteric HCO₃⁻ (Ion of NaHCO₃) are:

H₂CO₃ ⇄ HCO₃⁻ + H⁺ Ka1 -Here, HCO₃⁻ is acting as a base-

HCO₃⁻⇄ CO₃²⁻ + H⁺ Ka2 -Here, is acting as an acid-

Where Ka1 = 4.3x10⁻⁷ and Ka2 = 4.8x10⁻¹¹. As pKa = -log Ka:

pKa1 = 6.37; pKa2 = 10.32

As the pH of amphoteric salts is:

pH = (pKa1 + pKa2) / 2

pH = 8.34