To determine which of the following pairs represents a conjugate acid-base pair, we need to understand the concept of conjugate acid-base pairs in the Brønsted-Lowry theory. According to this theory, an acid is a substance that can donate a proton (H⁺), and a base is a substance that can accept a proton. When an acid donates a proton, it forms its conjugate base. Conversely, when a base accepts a proton, it forms its conjugate acid.
Let’s analyze each option given:
A. HCl and HBr
- HCl (hydrochloric acid) and HBr (hydrobromic acid) are both strong acids, but they do not differ by a single proton (H⁺). Therefore, they do not form a conjugate acid-base pair.
B. [tex]$H _2 S$[/tex] and [tex]$HS ^{-}$[/tex]
- When [tex]$H _2 S$[/tex] (hydrosulfuric acid) donates a proton (H⁺), it forms [tex]$HS ^{-}$[/tex] (hydrosulfide ion). Hence, [tex]$H _2 S$[/tex] and [tex]$HS ^{-}$[/tex] differ by one proton and thus form a conjugate acid-base pair.
[tex]$H _2 S \rightleftharpoons HS ^{-} + H^{+}$[/tex]
C. [tex]$NO _2$[/tex] and [tex]$NO _2$[/tex]-
- [tex]$NO _2$[/tex] (nitrogen dioxide) is not typically considered an acid or base in this context. [tex]$NO _2$[/tex] and [tex]$NO _2$[/tex]- (nitrite ion) do not differ by a single proton. Therefore, they do not form a conjugate acid-base pair.
D. HF and [tex]$H ^{+}$[/tex]
- HF (hydrofluoric acid) is a weak acid, but [tex]$H ^{+}$[/tex] (a proton) does not form a meaningful conjugate pair with HF in the way defined by the acid-base theory. Hence, they do not form a conjugate acid-base pair.
Given this analysis, the correct pair forming a conjugate acid-base pair is:
B. [tex]$H _2 S$[/tex] and [tex]$HS ^{-}$[/tex]
