Answer to Question #114266 in Chemistry for Amir

Question

when should I use 10 ^(2pH) in equilibrium concentration questions?

Answer 1

According to the definition, pH is the decimal logarithm of the reciprocal hydrogen ion concentration in a solution.

As a result, 10^-pH represents the concentration of the hydrogen ions.

According to the common reaction:

AH = A^- + H⁺

From here the equilibrium constant is:

K_eq = [A^-] [H⁺] / [AH]

If AH is a strong acid, it completely dissociates into [A^-] and [H⁺]. As a result, [A^-]=[H⁺]. From here:

K_eq = [H⁺]² / [AH]

As [H⁺] = 10^-pH, [H⁺]² = 10^-2pH.

As a result, 10^-2pH should be used if we have a strong acid.