The concentration of hydrogen ion is a measure of acidity or alkalinity of a solution…

The concentration of hydrogen ion is a measure of acidity or alkalinity of a solution.
For mono basic acids :
\(\mathrm{CH}_{3} \mathrm{COOH} \rightleftharpoons \mathrm{CH}_{3} \mathrm{COO}^{-}+\mathrm{H}^{+}\) 
\(\alpha=\sqrt{\frac{K_{a}}{C}}\) 
where, K_a = Dissociation constant of acid
C = molarity of acid
\(\left[\mathrm{H}^{+}\right]=\mathrm{C} \alpha,\left[\mathrm{H}^{+}\right]=\frac{\mathrm{K}_{\mathrm{a}}}{\alpha},\) \(\left[\mathrm{H}^{+}\right]=\sqrt{\mathrm{CK}_{\mathrm{a}}}\) 
pH of a weak acid can be calculated using [H⁺] by any of above methods.
\(\left[\mathrm{H}^{+}\right]_{\text {Total }}=\sqrt{\mathrm{C}_{1} \mathrm{~K}_{\mathrm{a}_{1}}+\mathrm{C}_{2} \mathrm{~K}_{\mathrm{a}_{2}}}\) (When two weak acids are mixed)
[H⁺] of polyprotic acid (weak, H₃A) (has dissociation constants \(\mathrm{K}_{\mathrm{a}_{1}} \gg \mathrm{~K}_{\mathrm{a}_{2}} \gg \mathrm{~K}_{\mathrm{a}_{3}}\)); will be contributed by first dissociation at the most.
 
What will be sulphide ion concentration of a dilute solution that has been saturated with 0.1 M H₂S if the pH of the solution is 3 ?
\(\mathrm{K}_{\mathrm{a}_{1}}=1 \times 10^{-7}\) ; \(\mathrm{K}_{\mathrm{a}_{2}}=1.3 \times 10^{-13}\)