Acid-Base Equations

The Survival Guide Acid-Base Equations

Type

Examples
Approximate Formulas
Strong Acid
HCl, HNO3, H2SO4 (first proton), HClO4
[H+] = CSA, (if CSA > 10-6)

pH = -log [H+]

Strong Base
NaOH, KOH, Ba(OH)2
[OH-] = CSB, (if CSB > 10-6)

[H+] = Kw/[OH-]

Weak Acid
Acids with a Ka, e.g. CH3COOH, or salts of weak bases, e.g. NH4Cl.

Weak Base
Bases with a Kb (mostly amines, e.g. NH3), or salts of weak acids, e.g. CH3COO-Na+.

Buffer
Mixtures of weak acids and their conjugate bases, e.g. CH3COOH mixed with CH3COO-Na+

Amphiprotic (polyprotic)
NaHCO3, KHPhthalate

Some Extensions to the formulas above:

The quadratic equation may need to be solved. For ax2+bx+c = 0,

For very dilute strong acids or bases, e.g. CHCl = 10-8 M HCl, charge balance gives us

[H+] = [Cl-] + [OH-], from which we get . Rearrange into standard quadratic equation form and solve for [H+].

For weak acids or bases, if successive approximation does not converge, convert the equation into the quadratic equation form and solve for [H+] or [OH-]

Dilute Buffers may require this extended equation:

The Full Amphiprotic Equation is:

David L. Zellmer, Department of Chemistry, California State University, Fresno. March 14, 1997