The pH of an acid in aqueous solution can be estimated based on knowledge of its dissociation constant and molarity.
NOTE: The following applies to monoprotic acids. A weak diprotic acid can be treated as a monoprotic acid with regards to hydronium ion concentration if the Ka1 value > 200 * Ka2 (page 49).
Strength of a Acid
Dissociation Constant (Kb)
weak
< 1
strong
>= 1
Once the concentration of hydronium [H3O+] is known for an acid, the pH can be calculated:
pH =
= -LOG10 [H+]
[H3O+] for a strong acid =
= (molarity of the acid in solution)
[H3O+] for a weak acid =
= 0.5 * (X – Ka)
X =
= SQRT((Ka^2) + (4 * Ka * (molarity of the acid in solution)))
where:
• The hydronium concentration is equal to the sum of all contributing chemical reactions. It is valid to drop the other contributing concentrations under certain circumstances (page 30). For example, very dilute solutions of a strong acid do not have a pH equal to the molarity of the acid since water dissociation becomes a factor.
• The equation for a weak acid is referred to as the quadratic formula.
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