# Equilibrium - Class 11 - Chemistry

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## Extra Questions - Equilibrium | NCERT | Chemistry | Class 11

Strongest conjugate base is:

(A) $\mathrm{Cl}^-$ (B) $\mathrm{Br}^-$ (C) $\mathrm{F}^-$ (D) $\mathrm{I}^-$

The correct option is **(C) $\mathrm{F}^-$**.

The strength of a conjugate base is inversely related to the strength of its corresponding acid. **Weaker acids have stronger conjugate bases**.

Considering the acids formed by addition of an $\mathrm{H}$ atom to each base, we have:

$\mathrm{HF}$

$\mathrm{HCl}$

$\mathrm{HBr}$

$\mathrm{HI}$

Among these, the $\mathrm{F}^-$ ion is the smallest in size, causing its electron density to be more concentrated around its nucleus. This results in a **short and stable $\mathrm{H}-\mathrm{F}$ bond**. The stability and short bond length make the $\mathrm{H}-\mathrm{F}$ bond difficult to break, indicating a challenging $\mathrm{H}^+$ ion donation process. Therefore, $\mathrm{HF}$ represents the weakest acid among the list. Consequently, its conjugate base **$\mathrm{F}^-$ is the strongest**.

#### Alternate Explanation:

The smallest size and highest electronegativity of $\mathrm{F}$ help it gain protons effectively, further supporting the character of $\mathrm{F}^-$ as the strongest conjugate base: $$ \mathrm{H}^+ + \mathrm{F}^- \rightarrow \mathrm{HF} $$

$X Y_{2}$ dissociates as $X Y_{2}(g) \rightleftharpoons X Y(g) + Y(g)$ when the initial pressure of $X Y_{2}$ is $600 , \mathrm{mm}$ of $\mathrm{Hg}$. The total equilibrium pressure is $800 , \mathrm{mm}$ of $\mathrm{Hg}$. Find the value of the equilibrium constant for the reaction, assuming that the volume of the system remains unchanged.

A) $50 , \mathrm{mm} , \mathrm{Hg}$

B) $100 , \mathrm{mm} , \mathrm{Hg}$

C) $166.6 , \mathrm{mm} , \mathrm{Hg}$

D) $400 , \mathrm{mm} , \mathrm{Hg}$

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