Anhydrous ${\text{AlC}}{{\text{l}}_3}$ is prepared by:
Which one of the following pairs of solutions is not an acidic buffer?
A. ${{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ and ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$
B. ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ and ${\text{N}}{{\text{a}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$
C. ${\text{HCl}}{{\text{O}}_{\text{4}}}$ and ${\text{NaCl}}{{\text{O}}_{\text{4}}}$
D. ${\text{C}}{{\text{H}}_{\text{2}}}{\text{COOH}}$ and ${\text{C}}{{\text{H}}_{\text{2}}}{\text{COONa}}$

Hint: An acid which does not dissociate completely in water is known as a weak acid. A base which dissociates completely in water is known as a strong base. Any solution that can donate or accept electrons and does not change its pH is known as a buffer solution. An acidic buffer is made up of weak acid and its salt with a strong base.

Complete step by step answer:
An acidic buffer is made up of weak acid and its salt with a strong base.
${{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ does not dissociate completely and thus, it is a weak acid and ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ is a salt of strong base ${\text{NaOH}}$. Thus, the pair ${{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ and ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ is an acidic buffer. Thus, option (A) is not correct.
${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ does not dissociate completely and thus, it is a weak acid and ${\text{N}}{{\text{a}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ is a salt of strong base ${\text{NaOH}}$. Thus, the pair ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ and ${\text{N}}{{\text{a}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ is an acidic buffer. Thus, option (B) is not correct.
${\text{HCl}}{{\text{O}}_{\text{4}}}$ dissociates completely and thus, it is not a weak acid but it is a strong acid and ${\text{NaCl}}{{\text{O}}_{\text{4}}}$ is a salt of strong base ${\text{NaOH}}$. Thus, the pair ${\text{HCl}}{{\text{O}}_{\text{4}}}$ and ${\text{NaCl}}{{\text{O}}_{\text{4}}}$ is not an acidic buffer. Thus, option (C) is correct.
${\text{C}}{{\text{H}}_{\text{2}}}{\text{COOH}}$ does not dissociate completely and thus, it is a weak acid and ${\text{C}}{{\text{H}}_{\text{2}}}{\text{COONa}}$ is a salt of strong base ${\text{NaOH}}$. Thus, the pair ${\text{C}}{{\text{H}}_{\text{2}}}{\text{COOH}}$ and ${\text{C}}{{\text{H}}_{\text{2}}}{\text{COONa}}$ is an acidic buffer. Thus, option (D) is not correct.
Thus, the pair ${\text{HCl}}{{\text{O}}_{\text{4}}}$ and ${\text{NaCl}}{{\text{O}}_{\text{4}}}$ is not an acidic buffer.

Thus, the correct option is (C) ${\text{HCl}}{{\text{O}}_{\text{4}}}$ and ${\text{NaCl}}{{\text{O}}_{\text{4}}}$.


Note:
Strong acids cannot form acidic buffers. As ${\text{HCl}}{{\text{O}}_{\text{4}}}$ is a strong acid it cannot form an acidic buffer.