Define pH. pH cannot be calculated directly from the molar concentration of a weak acid or weak base. Why? Derive an equation for the pH of a weak acid.

Explanation

The negative logarithm on value to the 10 of the hydrogen ion concentration expressed in moles/lit, in a solution is know as.pH. of the solution.
`pH:- log (H^(+))= "log"_(10)(1)/((H^(+)))`
`to` In case of a weak acid (or) weak base extent of ionisation is low so pH cannot be calculated directly from their molar concentrations.
Derivation of pH of a weak acid :
`to` A general step wise approch can be adopted to evaluate the pH of the weak acid as follows.
Step 1. The species present before dissociation are identified as Bronsted - Lowry, acids/ bases.
Step 2. Balanced equations for all possibel reactions i.e., with a species acting both as acid as well as base are written.
Step 3. The reaction with the higher `K_(a)` is identified as the primary reaction whilst the other is a - subsidiary reaction.
Step 4. Enlist in a tabular form the following values for each of the species in the primary reaction.
(a). Initial concentration. C.
(b) Change in concentration on proceeding to equilibrium in terms of a, degree of ionization,
Step 5. Substitute equilibrium concentrations into equilibrium constrant equation for principal reaction and solve for a. Step
6. Calculate the concentration of species.in principal raction. Step
7. Calculate `pH =-log[H_(3)O^(+)]`
The above mentioned methodology has been elucidated in the following examples.