Aqueous solutions of HNO3, KOH, CH3COOH, and CH3COONa of identical concentrations are provided. The pair(s) of solutions which form a buffer upon mixing are:

A buffer solution consists of a weak acid and its salt with a strong base or a weak base and its salt with a strong acid.

HNO3 is a strong acid, KOH is a strong base, CH3COOH is a weak acid, and CH3COONa is the salt of a weak acid (CH3COOH) and a strong base (NaOH).

Therefore, a buffer solution can be formed by mixing:

1. A weak acid (CH3COOH) and its salt with a strong base (CH3COONa). This is a mixture of CH3COOH and CH3COONa.
2. A weak acid and its conjugate base. CH3COOH is a weak acid, and CH3COO- (from CH3COONa) is its conjugate base. Thus CH3COOH and CH3COONa form a buffer.

Mixing HNO3 (strong acid) and CH3COOH (weak acid) will not form a buffer. Mixing HNO3 (strong acid) and CH3COONa (salt of weak acid and strong base) will result in a neutralization reaction, not a buffer. Mixing KOH (strong base) and CH3COONa (salt of weak acid and strong base) will not form a buffer either.