a
Given- \(K_a\) and \(c\)

To find \(-\left[H^{+}\right]=\)?

\(\quad\quad\quad\quad HA \rightleftharpoons H ^{+}+ A ^{-}\)

\(t=0 \quad\quad c \quad\quad\quad0\quad \quad 0\)

\(t=t_1 \quad c-x \quad\quad x \quad\quad x\)

For very weak acid \(\Rightarrow x\,<<<,1\)

\(K_a =\frac{\left[H^{+}\right]\left[A^{-}\right]}{[H A]}\)

\(=\frac{x \times x}{c-x}\)

where,\(K_a =\text { Dissociation censtent }\)

\(c =[H A]\)

\(x =H^{+} \text {concentration }\)

As \(\quad x\,<<<\,1\) so \(c-x \approx c\)

\(\therefore k_a =\frac{x^2}{c}\)

\(\Rightarrow x=\sqrt{k_a C}\)