Multiple Choice
What determines the pH of a solution formed when a salt dissociates in water and the resulting acid or base is weak?
17. Acid and Base Equilibrium
pH of Weak Bases
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What is the pH of a 0.400 M solution of NaF, given that the Kb of F- is 1.4 x 10^-11?
A
6.90
B
7.00
C
8.10
D
5.60
Verified step by step guidance1
Identify that NaF is a salt formed from a strong base (NaOH) and a weak acid (HF). Therefore, the solution will be basic due to the hydrolysis of F⁻ ions.
Write the hydrolysis reaction for F⁻: \( \text{F}^- + \text{H}_2\text{O} \rightleftharpoons \text{HF} + \text{OH}^- \).
Use the expression for the base dissociation constant \( K_b \) to find the concentration of OH⁻: \( K_b = \frac{[\text{HF}][\text{OH}^-]}{[\text{F}^-]} \).
Assume \([\text{OH}^-] = x\) and \([\text{HF}] = x\), and \([\text{F}^-] = 0.400 - x\). Substitute these into the \( K_b \) expression: \( 1.4 \times 10^{-11} = \frac{x^2}{0.400 - x} \).
Solve for \( x \) to find \([\text{OH}^-]\), then calculate the pOH using \( \text{pOH} = -\log[\text{OH}^-] \). Finally, find the pH using \( \text{pH} = 14 - \text{pOH} \).
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