Question

which of the following is not an acid-base conjugate pair?

• $\ce{h2co3}$ and $\ce{hco3-}$
• $\ce{nh4+}$ and $\ce{nh3}$
• $\ce{h2o}$ and $\ce{oh-}$
• $\ce{h2s}$ and $\ce{oh-}$
• none of the above

Explanation:

Step1: Recall conjugate pair rule

A conjugate pair differs by one H⁺.

Step2: Check each option

• H₂CO₃ (loses H⁺) → HCO₃⁻: valid
• NH₄⁺ (loses H⁺) → NH₃: valid
• H₂O (loses H⁺) → OH⁻: valid
• H₂S and OH⁻: no H⁺ difference

Step3: Identify invalid pair

H₂S and OH⁻ do not form a conjugate pair.

Answer:

H₂S and OH⁻