Definition of a Buffer Solution

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. It has the property of resisting a change in pH upon the addition of a small amount of a strong acid or a strong base.

An acidic buffer, relevant to this question, contains a weak acid and its conjugate base. This can be achieved in two primary ways:

1. By directly mixing a weak acid with its salt (which provides the conjugate base). For example, mixing $CH_3COOH$ (weak acid) and $CH_3COONa$ (its salt, providing the conjugate base $CH_3COO^-$).
2. By partial neutralization. For example:
   • Mixing a weak acid with a strong base, where the weak acid is in molar excess. The reaction forms the conjugate base, leaving some of the weak acid unreacted.
   • Mixing the salt of a weak acid (the conjugate base) with a strong acid, where the salt is in molar excess. The reaction forms the weak acid, leaving some of the conjugate base unreacted.

Given solutions are: $HNO_3$ (strong acid), $KOH$ (strong base), $CH_3COOH$ (weak acid), and $CH_3COONa$ (salt of weak acid, provides conjugate base $CH_3COO^-$).

Let's evaluate each option:

A: $HNO_3$ and $CH_3COOH$

1. $HNO_3$ is a strong acid.
2. $CH_3COOH$ is a weak acid.
3. Mixing a strong acid and a weak acid results in a solution containing both acids. There is no significant amount of a conjugate base present. The dissociation of the weak acid ($CH_3COOH$) is suppressed due to the common ion effect from $H^+$ provided by the strong acid ($HNO_3$). This mixture is not a buffer.

B: $KOH$ and $CH_3COONa$

1. $KOH$ is a strong base.
2. $CH_3COONa$ provides the acetate ion ($CH_3COO^-$), which is a weak base (the conjugate base of $CH_3COOH$).
3. Mixing a strong base and a weak base results in a strongly basic solution. There is no weak acid component to form a buffer pair. This mixture is not a buffer.

C: $HNO_3$ and $CH_3COONa$

1. $HNO_3$ is a strong acid.
2. $CH_3COONa$ provides the conjugate base, $CH_3COO^-$.
3. When mixed, they react as follows: $HNO_3(aq) + CH_3COONa(aq) \rightarrow CH_3COOH(aq) + NaNO_3(aq)$.
4. The net ionic equation is: $H^+(aq) + CH_3COO^-(aq) \rightarrow CH_3COOH(aq)$.
5. To form a buffer, the final solution must contain both the weak acid ($CH_3COOH$) and its conjugate base ($CH_3COO^-$). This can be achieved if the strong acid ($HNO_3$) is the limiting reactant, i.e., the initial moles of $CH_3COONa$ are greater than the initial moles of $HNO_3$.
6. For example, if we mix 2 moles of $CH_3COONa$ with 1 mole of $HNO_3$, the reaction will produce 1 mole of $CH_3COOH$, and 1 mole of $CH_3COONa$ will remain unreacted. The resulting solution contains a mixture of $CH_3COOH$ and $CH_3COONa$, which is an acidic buffer.
7. Since it is possible to form a buffer by mixing these two solutions in appropriate proportions, this option is correct.

D: $CH_3COOH$ and $CH_3COONa$

1. $CH_3COOH$ is a weak acid.
2. $CH_3COONa$ is the salt of this weak acid with a strong base, and it provides the conjugate base $CH_3COO^-$.
3. Mixing a weak acid and its conjugate base is the direct method of preparing a buffer solution. The resulting solution will contain both species in concentrations dependent on the mixing ratio.
4. This mixture is a classic example of an acidic buffer. Therefore, this option is correct.