To determine which pair of substances cannot function as a buffer system in aqueous solution, let's first understand what constitutes a buffer system.
A buffer system usually consists of a weak acid and its conjugate base, or a weak base and its conjugate acid. The purpose of a buffer is to maintain pH stability by neutralizing added acids or bases.
Now, let's analyze each pair of substances:
1. [tex]$NaH_2PO_4$[/tex] and [tex]$Na_2HPO_4$[/tex]:
- [tex]$NaH_2PO_4$[/tex] is sodium dihydrogen phosphate (a weak acid).
- [tex]$Na_2HPO_4$[/tex] is disodium hydrogen phosphate (the conjugate base of [tex]$NaH_2PO_4$[/tex]).
- This pair can function as a buffer system.
2. [tex]$NH_3$[/tex] and [tex]$NH_4Cl$[/tex]:
- [tex]$NH_3$[/tex] (ammonia) is a weak base.
- [tex]$NH_4Cl$[/tex] (ammonium chloride) provides [tex]$NH_4^+$[/tex], which is the conjugate acid of [tex]$NH_3$[/tex].
- This pair can function as a buffer system.
3. [tex]$NaHCO_3$[/tex] and [tex]$Na_2CO_3$[/tex]:
- [tex]$NaHCO_3$[/tex] is sodium bicarbonate (a weak acid or amphoteric substance).
- [tex]$Na_2CO_3$[/tex] is sodium carbonate (the conjugate base of [tex]$HCO_3^-$[/tex]).
- This pair can function as a buffer system.
4. NaOH and [tex]$NH_4OH$[/tex]:
- NaOH (sodium hydroxide) is a strong base.
- [tex]$NH_4OH$[/tex] (ammonium hydroxide) is a weak base.
- Both are bases, and they do not constitute a conjugate acid-base pair. Therefore, they cannot function as a buffer system.
5. HCN and NaCN:
- HCN (hydrocyanic acid) is a weak acid.
- NaCN provides CN^-, which is the conjugate base of HCN.
- This pair can function as a buffer system.
Among the given pairs, the pair that could not function as a buffer system is NaOH and [tex]$NH_4OH$[/tex].
Hence, the correct answer is:
4
