Henderson Hasselbalch Equation

Consider the ionization of some weak acid, HA, occurring with an acid dissociation constant, Ka. Then,

Rearranging this expression in terms of the parameter of interest, [H+], we have

Taking the logarithm of both sides gives log [H+] = log Ka + logio [HA]

If we change the signs and define pKa = -log Ka, we have

This relationship is known as the Henderson-Hasselbalch equation. Thus, the pH of a solution can be calculated, provided Ka and the concentrations of the weak acid HA and its conjugate base A- are known. Note particularly that when [HA] = [A-], pH = pKa. For example, if equal volumes of 0.1 M HAc and 0.1 M sodium acetate are mixed, then pH = pKa = 4.76

(Sodium acetate, the sodium salt of acetic acid, is a strong electrolyte and dissociates completely in water to yield Na+ and Ac-.)

%

HCOOH (formic acid)

1.78 >

< 10"

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