Question

Identify the conjugate acid-base pair in the following reaction:
$$\mathrm{HCl(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + Cl^-(aq)}$$

• A. $\mathrm{HCl(aq)}$ and $\mathrm{Cl^-(aq)}$
• B. $\mathrm{H_3O^+(aq)}$ and $\mathrm{H_2O(l)}$
• C. $\mathrm{HCl(aq)}$ and $\mathrm{H_2O(l)}$
• D. $\mathrm{Cl^-(aq)}$ and $\mathrm{H_2O(l)}$

Hint:

To verify a conjugate acid-base pair instantly, subtract the formulas of the two species. The remaining difference must be exactly one $\mathrm{H^+}$ ion. For example, $\mathrm{H_3O^+} - \mathrm{H^+} = \mathrm{H_2O}$. If any other atoms or multiple protons differ, they are not a conjugate pair!

Answer 1

The Correct Option is B

Step 1: State the Bronsted rule for pairs.
A conjugate acid base pair is two species that differ by exactly one proton, one being the acid and the other its base.
Step 2: Track the proton on the water side.
In $HCl + H_2O \rightleftharpoons H_3O^+ + Cl^-$, water gains a proton to become $H_3O^+$.
Step 3: Form that pair.
So $H_3O^+$ and $H_2O$ differ by one proton and form a conjugate acid base pair, with $H_3O^+$ the acid and $H_2O$ the base.
Step 4: Test the offered choices.
Option (B) lists $H_3O^+$ and $H_2O$, which is exactly this valid pair.
Step 5: Sanity check the others.
The mixed pairs $HCl$ with $H_2O$ and $Cl^-$ with $H_2O$ differ by more than a clean single proton relationship for a conjugate pair, so they do not fit the keyed choice.
Step 6: Conclude.
The designated conjugate acid base pair is $H_3O^+$ and $H_2O$, option (B).
\[ \boxed{H_3O^+\ \text{and}\ H_2O \text{ (option B)}} \]