Question

The ionisation constant of the weak acid HF whose concentration is 0.1 M is \( 3.5 \times 10^{-4} \). The equilibrium constant value for the reaction
\[ \text{F}^- + \text{H}_2\text{O} \leftrightarrow \text{HF} + \text{OH}^- \]
is ............ and the pH of the aqueous solution of the weak acid is ...............

• A. \( K_{\text{eq}} = 4.4 \times 10^{-11} \quad \text{pH} = 4.25 \)
• B. \( K_{\text{eq}} = 3.58 \times 10^{-11} \quad \text{pH} = 4.19 \)
• C. \( K_{\text{eq}} = 2.86 \times 10^{-11} \quad \text{pH} = 3.23 \)
• D. \( K_{\text{eq}} = 3.92 \times 10^{-10} \quad \text{pH} = 4.07 \)

Hint:

When dealing with weak acid dissociation and equilibrium constants, remember that the dissociation of water and the conjugate base affect the equilibrium. Also, the pH can be calculated from the \( [\text{H}^+] \) concentration using \( \text{pH} = -\log [\text{H}^+] \).

Answer 1

The Correct Option is C