(i) To resist changes in its pH on the addition of an acid (or) a base, the buffer solution should contain both acidic as well as basic components so as to neutralize the effect of added acid (or) base and at the same time, these components should not consume each other.
(ii) Let us explain the buffer action in a solution containing CH₃COOH and CH₃COONa.
(iii) The dissociation of the buffer components occurs as below.
\(\mathrm{CH}_{3} \mathrm{COOH}_{(\mathrm{aq})} \rightleftharpoons \mathrm{CH}_{3}-\mathrm{COO}_{(\mathrm{aq})}^{-}+\mathrm{H}_{3} \mathrm{O}_{(\mathrm{aq})}^{+} \)
\(\mathrm{CH}_{3} \mathrm{COONa}_{(\mathrm{s})} \stackrel{\mathrm{H}_{2} \mathrm{O}_{(\mathrm{l})}}{\longrightarrow} \mathrm{CH}_{3}-\mathrm{COO}_{(\mathrm{aq})}^{-}+\mathrm{Ha}_{(\mathrm{aq})}^{+}\)

Henderson equation is an equation which is used to determine the pH of an acid buffer with the help of the dissociation constant K_a of the weak acid and concentration of the acid and the salt used.
\(\boxed { pH={ pK }_{ a }+\log\frac { [salt] }{ [acid] } }\)
For a basic buffer \(pH={ pK }_{ b }+\log\frac { [salt] }{ [base] } \)

Solubility can be calculated from the molar solubility i.e., the maximum number of moles of solute that can be dissolved in one litre of the solution.
For a solute X_mY_n,
\({ X }_{ m }{ Y }_{ n(s) }\rightleftharpoons m{ X }_{ (aq) }^{ n+ }+{ nY }_{ (aq) }^{ m- }\)
From the above stoichiometrically balanced equation we have come to know that 1 mole of X_m Y_n(s) dissociated to furnish 'm' moles of Xⁿ⁺ and 'n' moles of Y^m- if 's' is molar solubility of X_mY_n then
[Xⁿ⁺] = ms and [Y^m-] = ns
ஃK_sp = [Xⁿ⁺]^m [Y^m-]ⁿ
K_sp = (ms)^m (ns)ⁿ
K_sp = (m)^m (n)ⁿ (s)^m+n

Buffer index β, as a quantitative measure of the buffer capacity. It is defined as the number of gram equivalents of acid or base added to 1litre of the buffer solution to change its pH by unity.
\(\beta =\frac { dB }{ d(pH) } \)
Here,
dB = number of gram equivalents of acid / base added to one litre of buffer solution
d(pH) = The change in the pH after the addition of acid / base.

Pure water itself has a little tendency to dissociate. i.e, one water molecule donates a proton to an another water molecule. This is known as auto ionisation of water and it is represented as below.