Question

Identify $\text{base}_2$ for the following equation according to Br\o{}nsted--Lowry theory: \[HCl_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + Cl^-_{(aq)}\]}

• A. $H_3O^+_{(aq)}$
• B. $H_2O_{(l)}$
• C. $Cl^-_{(aq)}$
• D. $HCl_{(aq)}$

Hint:

Conjugate acid--base pairs always differ by exactly one $H^+$ ion. Acid $\rightarrow$ Base$_2$ (loses $H^+$); Base $\rightarrow$ Acid$_2$ (gains $H^+$).

Answer 1

The Correct Option is C

Step 1: Concept
According to Br\o{}nsted--Lowry theory, an acid donates a proton ($H^+$) and a base accepts a proton. Each acid--base pair is related by the gain or loss of one proton.

Step 2: Meaning
The conjugate pairs are: $\text{Acid}_1 / \text{Base}_2$ and $\text{Base}_1 / \text{Acid}_2$.

Step 3: Analysis
In the given equation: $HCl$ donates $H^+$ to become $Cl^-$ $\Rightarrow$ $HCl = \text{Acid}_1$, $Cl^- = \text{Base}_2$. $H_2O$ accepts $H^+$ to become $H_3O^+$ $\Rightarrow$ $H_2O = \text{Base}_1$, $H_3O^+ = \text{Acid}_2$.

Step 4: Conclusion
$\text{Base}_2$ is the conjugate base of $HCl$, which is $Cl^-$. Final Answer: (C)