Acids are divided into two categories based on the ease with which they can donate protons to the solvent: i) strong acids and ii) weak acids Strong acids are acids that completely dissociate in water. The reaction of an acid with its solvent (typically H2O) is called an acid dissociation reaction. Weak acids are acids that dissociate partially in water. The extent of dissociation is given by the equilibrium constant. Note: A measure of the relative strength of an acid is: i) the equilibrium constant ka of the dissociation reaction of the acid in water (depends on temperature) ii) the degree of dissociation α of the acid in water (depends on the concentration of the acid an on temperature).

1. Strong and weak acids and bases
(Please see page 4 of this article for clickable links)
Acids are divided into two categories based on the ease with which they can
donate protons to the solvent: i) strong acids and ii) weak acids
Strong acids are acids that completely dissociate in water. The reaction of an
acid with its solvent (typically H2O) is called an acid dissociation reaction.
Strong acids, such as HNO3, almost completely transfer their protons to the
solvent molecules:
HNO3(aq) + H2O(l) → H3O+
(aq) + NO3
–
(aq)
In this reaction H2O serves as the base. The hydronium ion, H3O+
, is the
conjugate acid of H2O, and the nitrate ion is the conjugate base of HNO3. It is
the hydronium ion H3O+
that is the acidic species in solution, and its
concentration determines the acidity of the resulting solution.
The common strong acids in aqueous solutions are given in Table I.1:
Table I.1: Strong acids in aqueous solutions
When a solution of 0,1 M HNO3 dissolves in water dissociates completely to
0,1 M H+
and 0,1 M NO3
–
.
Weak acids, for example acetic acid, cannot completely donate their acidic
protons to the solvent. Instead, most of the acid remains undissociated, with
only a small fraction present as the conjugate base (CH3COO–
).

2. Weak acids are acids that dissociate partially in water. The extent of
dissociation is given by the equilibrium constant.
Note:
A measure of the relative strength of an acid is: i) the equilibrium
constant ka of the dissociation reaction of the acid in water (depends on
temperature) ii) the degree of dissociation α of the acid in water
(depends on the concentration of the acid an on temperature).
The equilibrium constant for this reaction is called an acid dissociation
constant, ka, and is written as:
ka = [H3O+
] * [CH3COO–
] / [CH3COOH] = 1.76 * 10-5
The magnitude of ka provides information about the relative strength of a
weak acid, with a smaller ka corresponding to a weaker acid. The opposite,
small pka values characterize stronger acids.
Table I.2 below gives the ka and pka values for a number of commonly-
encountered weak acids (25 ∘C).
Compound ka pka
Acetic acid (CH3COOH) 1.76 x 10-5
4.75
Adipic acid
(CH2)4(COOH)2
3.71 x 10-5
Step 1
3.87 x 10-5
Step 2
4.43
4.41
Benzoic acid 6.46 x 10-5
4.19

3. (C6H5COOH)
Carbonic acid (H2CO3) 4.3 x 10-7
5.61 x 10-11
6.37
10.25
Chloroacetic acid
(ClCH2CO2H)
1.4 x 10-3
2.85
Chromic acid (H2CrO4) 1.8 x 10-1
3.2 x 10-7
0.74
6.49
Formic acid (HCOOH) 1.77 x 10-4
(20 ○
C) 3.75
Hydrocyanic acid (HCN) 4.93 x 10-10
9.31
Hydrofluoric acid (HF) 3.53 x 10-4
3.45
Hypobromous acid
(HBrO)
2.06 x 10-9
8.69
Hypochlorous acid
(HClO)
2.95 x 10-8
7.53
Hypoiodous acid (HIO) 2.3 x 10-11
10.64
Iodic acid (HIO3) 1.69 x 10-1
0.77
Maleic acid 1.42 x 10-2
8.57 x 10-7
1.83
6.07
Nitrous acid (HNO2) 4.6 x 10-4
(12,5 ○
C) 3.37
Oxalic acid (H2C2O4) 5.90 x 10-2
6.40 x 10-5
1.23
4.19
Periodic acid 2.3 x 10-2
1.64
Phenol (C6H5OH) 1.28 x 10-10
9.89
o-Phosphoric acid
(H3PO4)
7.52 x 10-3
6.23 x 10-8
2.12
7.21
Table I.2: The ka and pka values for a number of commonly-
encountered weak acids
Simirarly, strong bases, such as NaOH, are bases that completely dissociate in
water to produce hydroxyl ion OH-
:
NaOH → Na+
+ OH-

4. When 0.1M NaOH dissolves in H2O dissociates to 0.1M Na+ and 0.1M OH-
.
The common strong bases in aqueous solutions are given in Table I.3:
Strong bases in aqueous solutions
Group 1A metal
hydroxides
LiOH, NaOH, KOH, RbOH, CsOH
Group 2A metal hydroxides Ca(OH)2, Sr(OH)2, Ba(OH)2
Table I.3: Strong bases in water
For more info on strong acids and bases and acid-base equilibria
please see the following links:
Strong acid and bases - Weak acid and bases - Dissociation constants and
pK's
pH of strong acids - Examples
Ionic Equilibrium - Strong Acids and Bases – A general relation for the pH of a
strong acid.
Weak Acids and Bases – Calculate the pH of a weak acid (a general equation)