Question

choose the case that is not a conjugate acid-base pair.

1. $\ce{hco3^-}$, $\ce{co3^{2-}}$
2. $\ce{h3o^+}$, $\ce{h2o}$
3. $\ce{oh^-}$, $\ce{o^{2-}}$
4. $\ce{h3po4}$, $\ce{po4^{3-}}$
5. $\ce{nh2oh2^+}$, $\ce{nh2oh}$

Explanation:

Step1: Recall conjugate acid-base pair definition

A conjugate acid - base pair differs by one proton ($\ce{H+}$).

Step2: Analyze option 1

For $\ce{HCO3-}$ and $\ce{CO3^{2-}}$: $\ce{HCO3-
ightleftharpoons H+ + CO3^{2-}}$. They differ by one $\ce{H+}$, so they are a conjugate acid - base pair.

Step3: Analyze option 2

For $\ce{H3O+}$ and $\ce{H2O}$: $\ce{H3O+
ightleftharpoons H+ + H2O}$. They differ by one $\ce{H+}$, so they are a conjugate acid - base pair.

Step4: Analyze option 3

For $\ce{OH-}$ and $\ce{O^{2-}}$: $\ce{OH-
ightleftharpoons H+ + O^{2-}}$. They differ by one $\ce{H+}$, so they are a conjugate acid - base pair.

Step5: Analyze option 4

For $\ce{H3PO4}$ and $\ce{PO4^{3-}}$: $\ce{H3PO4}$ would lose three protons to form $\ce{PO4^{3-}}$ ($\ce{H3PO4
ightleftharpoons 3H+ + PO4^{3-}}$). They differ by three protons, not one. So they are not a conjugate acid - base pair.

Step6: Analyze option 5

For $\ce{NH2OH2+}$ and $\ce{NH2OH}$: $\ce{NH2OH2+
ightleftharpoons H+ + NH2OH}$. They differ by one $\ce{H+}$, so they are a conjugate acid - base pair.

Answer:

1. $\ce{H3PO4}$, $\ce{PO4^{3-}}$