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Q.
The $pH$ of $1\, N$ aqueous solutions of $HCl,\, CH_{3}COOH$ and $HCOOH$ follows the order
KVPYKVPY 2015Equilibrium
Solution:
According to Bronsted-Lowry concept of acids and bases, more is the stability of the conjugate base stronger will be the acid. The conjugate bases of the given solutions are :
$HCl \rightleftharpoons H^{+} + \underset{\text{Conjugate base}}{CI^{-}}$
$HCOOH \rightleftharpoons \underset{\text{Conjugate base} }{HCOO^{-}} +H^+$
$HC_{3}COOH \rightleftharpoons \underset{\text{Conjugate base}}{CH_{3}COO^{-}} +H^-$
Stability of conjugate base follows the order
Thus, the acidic strength follows the same order, i.e.
$HCl > HCOOH > CH_{3}COOH$
We also know that $pH$ is inversely proportional to the concentration of $H^{+}$ ions (acidic strength)
i.e $pH\propto\frac{1}{\left[H^{+}\right]}$
Thus, the pH of $1\,N$ aqueous solution follows the order
$CH_{3}COOH > HCOOH >HCI$
