Question

We have acidic buffer of $C{H}_{3}COONa$ and $C{H}_{3}COOH$. One or more of the following operations will not change pH
I : diluting the mixture ten times
II : adding some HCl
III : adding some NaOH
IV : adding equal moles of $C{H}_{3}COONa$ and $C{H}_{3}COOH$ into the buffer.
Select correct alternate.

• A. I, II, III and IV
• B. II and III
• C. I and IV
• D. II and IV

Answer 1

The correct option is A I, II, III and IV

For given buffer, $pH=p{K}_a+\log \frac{\left[C{H}_{3}COONa\right]}{\left[C{H}_{3}COOH\right]}$

(I) Diluting will change the concentrations such that the ratio remains the same. Hence $pH$ will not change.

(II) & (III) Adding some $HCl$ or $NaOH$ will change the concentration ratio, thus changing the $pH$.

(IV) Adding equal moles of $C{H}_{3}COONa$ & $C{H}_{3}COOH$ may change the ratio, thus changing the $pH$ .