Can you plz explain why this is...acid/base question?

Question

The salt of a weak acid , as well as the acid itself, must be present to form a buffer solution b/c

- the anion from the salt is needed to partially neutralize added acid

why is that the right answer?

Answer 1

Buffer solutions are solutions that resist change in hydronium ion and the hydroxide ion concentration (and consequently pH) upon addition of small amounts of acid or base, or upon dilution. Buffer solutions consist of a weak acid and its conjugate base (more common) or a weak base and its conjugate acid (less common). The resistive action is the result of the equilibrium between the weak acid (HA) and its conjugate base (A−):

HA(aq) + H2O(l) ↔ H3O+(aq) + A−(aq)

Any alkali added to the solution is consumed by the hydronium ions. These ions are mostly regenerated as the equilibrium moves to the right and some of the acid dissociates into hydronium ions and the conjugate base. If a strong acid is added, the conjugate base is protonated, and the pH is almost entirely restored. This is an example of Le Chatelier's principle and the common ion effect. This contrasts with solutions of strong acids or strong bases, where any additional strong acid or base can greatly change the pH. This may be easier to see by comparing two graphs when an strong acid is titrated with a strong base the curve will have a large gradient throughout showing that a small addition of base/acid will have a large effect compared to a weak acid/strong base titration curve which will have a smaller gradient near the pKa.

Illustration of buffering effect: Sodium acetate/acetic acid buffer, as you ask about:

The acid dissociation constant for acetic acid-sodium acetate is given by the equation:

Ka = [H+] [ CH3COO-] / CH3COOH



Since this equilibrium only involves a weak acid and base, it can be assumed that ionization of the acetic acid and hydrolysis of the acetate ions are negligible. In a buffer consisting of equal amounts of acetic acid and sodium acetate, the equilibrium equation simplifies to

Ka = [H + ],

and the pH of the buffer as is equal to the pKa.

To determine the effect of addition of a strong acid such as HCl, the following mathematics would provide the new pH. Since HCl is a strong acid, it is completely ionized in solution. This increases the concentration of H+ in solution, which then neutralizes the acetate by the following equation.

CH3COO- + H+→ CH3COOH



The consumed hydrogen ions change the effective number of moles of acetic acid and acetate ions:

Moles of CH3COO-  =  initial moles of CH3COO-  - initial moles of HCl

Moles of CH3COOH  =  initial moles of CH3COOH + initial moles of HCl

After accounting for volume change to determine concentrations, the new pH could be calculated . Any neutralization will result in a small change in pH, since pH is on a logarithmic scale.

Answer 2

..Because the anion tht is to say only the -ve ion is required to neutralize e.g.

CH3COOH --> only CH3COO is required

now a salt of this acid would be NaCH3COO which is also a source of CH3Coo

Now weak acid and the anion added up will equate to the power of the strong base

hope tht helps .....

Answer 3

CH3COONa and CH3COOH   form a buffer solution.it is for maintain the PH of solution.