Question

We have acidic buffer of CH₃COONa and CH₃COOH . One or more of the following operations will not change pH

I: diluting the mixture ten times
II : adding some HCI
III : adding some NaOH
IV : adding equal moles of CH₃COONa and CH₃COOH 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

Only diluting the buffer and adding equal moles of acetic acid and sodium acetate (operations I and IV) will not change the pH of an acidic buffer consisting of acetic acid and sodium acetate. C. I and IV

Step-by-step explanation:

The question revolves around the principle of how an acidic buffer, made of acetic acid (CH₃COOH) and sodium acetate (CH₃COONa), behaves when exposed to various perturbations. Out of the four operations listed, the operations that would not change the pH of the buffer are diluting the mixture and adding equal moles of acetic acid and sodium acetate to the buffer, which are options I and IV respectively.

When a buffer solution is diluted, the ratio of the concentration of the acid and its conjugate base remains the same, which means the pH remains unchanged. Likewise, adding equal amounts of acetic acid and sodium acetate maintains the ratio, thereby preserving the pH. However, adding strong acid (HCl) or strong base (NaOH) directly interacts with the buffer components, altering concentrations and potentially changing the pH, therefore, operations II and III would change the pH.

Answer 2

Dilution of a buffer and the addition of equal moles of buffer components do not alter the pH, while adding strong acids or bases does. Therefore, the correct option is C. I and IV.

Step-by-step explanation:

The ability of a buffer to maintain its pH upon dilution or the addition of small amounts of acid or base lies in the equilibrium between the weak acid and its conjugate base. When a buffer is diluted, the ratio of the weak acid to its conjugate base does not change, so the pH remains constant. Thus, diluting the mixture (option I) will not change the pH. Adding a strong acid (option II) will shift the equilibrium, as the excess H+ ions will react with the conjugate base (CH3COO-) of the buffer, making more of the weak acid (CH3COOH) and reducing the pH.

Likewise, adding a strong base (option III) will consume the H+ ions released by the weak acid in the buffer, forming water and the conjugate base (CH3COO-), which would slightly increase the pH. However, if equal moles of CH3COONa (the salt/conjugate base) and CH3COOH (the weak acid) are added, the ratio of these buffer components remains the same, which means no significant pH change (option IV). Therefore, operations I and IV will not change the pH, while operations II and III will. The correct option is C. I and IV.