|Weak acids, like weak bases, are equilibrium mixtures,
of a higher concentration of reactant molecules than product ions.
A weak acid like acetic acid is written as:
H2O + CH3COOH <-------> C2H3O2-
the acid hydrogen
|The H at the start of the formula is termed the acid
since it is this H atom which reacts with the water to form the acidic
properties of the solution. You know it as the H on the -COOH portion
the carboxylic acid in organic acids.
|In solution, weak acids have a very low degree of
eg. a 0.10 mol/L solution of HCl is 100% dissociated, whereas a 0.10
solution of acetic acid is only about 1.3% dissociated.
HCl + H2O
before 0.10 M 0 M 0 M
0.1 M 0.1 M
|For acetic acid we need some equations:
CH3COOH + H2O <-------> H3O+
|You can see from the equation that the moles of acetic
= the moles of H3O+ formed which is also equal
the moles of CH3COO- formed.
|Since only 1.3% of the 0.10 mol/L dissociate then the 
|1.3% * 0.10 mol CH3COOH = 0.0013
H3O+= 0.0013 mol CH3COO-
100 1 L solvent 1 L 1 L
|A 0.60 mol/L solution of formic acid, HCHO2, is 1.8% dissociated. Calculate the  of each species (excluding the water) in the equilibrium system|