Equilibrium Ques 81
81. Buffer solutions have constant acidity and alkalinity because
[2012]
(a) these give unionised acid or base on reaction with added acid or alkali.
(b) acids and alkalies in these solutions are shielded from attack by other ions.
(c) they have large excess of $H^{+}$or $OH^{-}$ions
(d) they have fixed value of $pH$
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Solution:
- (a) Constant acidity and alkalinity of buffer solution is due to the unionised acid or base.
Lets take an example of an acidic buffer $CH_3 COOH$ and $CH_3 COONa$.
$CH_3 COOH \rightarrow CH_3 COO^{-}+H^{+}$;
$CH_3 COONa \rightarrow CH_3 COO^{-}+Na^{+}$
when few drops of $HCl$ are added to this buffer, the $H^{+}$of $HCl$ immediatly combine with $CH_3 COO^{-}$ions to form undissociated acetic acid molecules. Thus there will be no $H^{+}$ions to combine with $CH_3 COO^{-}$ions to form undissociated acetic acid molecules. Thus there will be no appreciable change in its $pH$ value. Likewise if few drops of $NaOH$ are added, the $OH^{-}$ions will combine with $H^{+}$ions to form unionised water molecule. Thus $pH$ of solution will remain constant.
BETA
