Question

which is not a conjugate acid-base pair?
$\ce{h3o+}$ and $\ce{h2o}$
$\ce{h2o}$ and $\ce{oh-}$
$\ce{nh3}$ and $\ce{nh2^{--}}$

Explanation:

Step1: Recall conjugate acid - base pair definition

A conjugate acid - base pair differs by one proton ($\ce{H+}$). The acid donates a proton to form its conjugate base, and the base accepts a proton to form its conjugate acid.

Step2: Analyze $\ce{H3O+}$ and $\ce{H2O}$

$\ce{H3O+}$ can donate a proton to form $\ce{H2O}$: $\ce{H3O+
ightarrow H2O + H+}$. So $\ce{H3O+}$ (acid) and $\ce{H2O}$ (conjugate base) are a conjugate acid - base pair.

Step3: Analyze $\ce{H2O}$ and $\ce{OH-}$

$\ce{H2O}$ can donate a proton to form $\ce{OH-}$: $\ce{H2O
ightarrow OH- + H+}$. So $\ce{H2O}$ (acid) and $\ce{OH-}$ (conjugate base) are a conjugate acid - base pair.

Step4: Analyze $\ce{NH3}$ and $\ce{NH2^{2-}}$

For a conjugate acid - base pair, the difference should be one $\ce{H+}$. $\ce{NH3}$ has 3 $\ce{H}$ atoms, $\ce{NH2^{2-}}$ has 2 $\ce{H}$ atoms and a charge difference of 2 (from 0 to - 2). The difference in protons is 1, but the charge difference is not consistent with a single proton transfer. Let's check the proton transfer: If $\ce{NH3}$ donates a proton, it should form $\ce{NH2-}$ (since $\ce{NH3 - H+
ightarrow NH2-}$), not $\ce{NH2^{2-}}$. The difference between $\ce{NH3}$ and $\ce{NH2^{2-}}$ is not just one proton (it's one proton and an extra electron - like charge change which is not from a single proton transfer).

(Assuming the last option is $\ce{NH4+}$ and $\ce{NH3}$: $\ce{NH4+}$ donates a proton to form $\ce{NH3}$: $\ce{NH4+
ightarrow NH3 + H+}$, so they are a conjugate acid - base pair. But focusing on the given option $\ce{NH3}$ and $\ce{NH2^{2-}}$)

Answer:

$\ce{NH3}$ and $\ce{NH2^{2-}}$ (the option with $\ce{NH3}$ and $\ce{NH2^{2-}}$)