Identification of Buffer Pair Solutions Upon Mixing

An acidic buffer solution typically consists of a weak acid and its conjugate base (a salt of the weak acid with a strong base).

Let the molar concentration of all the provided solutions be $C$.

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Analysis of the Options:

1. Option A: $\text{HNO}_3$ and $\text{CH}_3\text{COOH}$

   • $\text{HNO}_3$ is a strong acid and $\text{CH}_3\text{COOH}$ is a weak acid.
   • A mixture of a strong acid and a weak acid does not contain a conjugate acid-base pair. Therefore, it cannot act as a buffer solution.

2. Option B: $\text{KOH}$ and $\text{CH}_3\text{COONa}$

   • $\text{KOH}$ is a strong base and $\text{CH}_3\text{COONa}$ is a salt of a weak acid and a strong base.
   • Mixing a strong base with a salt does not generate any weak acid or conjugate pair. Hence, it cannot form a buffer solution.

3. Option C: $\text{HNO}_3$ and $\text{CH}_3\text{COONa}$

   • When a strong acid ($\text{HNO}_3$) is added to the salt of a weak acid ($\text{CH}_3\text{COONa}$), a neutralization reaction occurs:
     $\text{CH}_3\text{COONa (aq)} + \text{HNO}_3\text{ (aq)} \rightarrow \text{CH}_3\text{COOH (aq)} + \text{NaNO}_3\text{ (aq)}$
   • If the volume of $\text{CH}_3\text{COONa}$ solution ($V_1$) is greater than the volume of $\text{HNO}_3$ solution ($V_2$), i.e., $V_1 > V_2$:
     • Initial moles: $n(\text{CH}_3\text{COONa}) = C \cdot V_1$ and $n(\text{HNO}_3) = C \cdot V_2$.
     • After reaction, moles remaining:
       $\text{Moles of } \text{CH}_3\text{COOH formed} = C \cdot V_2$
       $\text{Moles of } \text{CH}_3\text{COONa remaining} = C \cdot (V_1 - V_2) > 0$
       $\text{Moles of } \text{HNO}_3 \text{ remaining} = 0$
   • The resulting mixture contains both $\text{CH}_3\text{COOH}$ (weak acid) and $\text{CH}_3\text{COONa}$ (its conjugate base), which forms an acidic buffer solution.

4. Option D: $\text{CH}_3\text{COOH}$ and $\text{CH}_3\text{COONa}$

   • $\text{CH}_3\text{COOH}$ is a weak acid and $\text{CH}_3\text{COONa}$ is its salt containing the conjugate base $\text{CH}_3\text{COO}^-$.
   • Mixing these two solutions directly yields a solution containing a weak acid and its salt, which forms an acidic buffer solution for any non-zero mixing ratio.

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Conclusion:

The pair(s) of solutions that can form a buffer upon mixing are C and D.