Multiple Choice
What is the theoretical pH of a 0.1 M solution of sodium acetate (NaC2H3O2), a weak base?
17. Acid and Base Equilibrium
pH of Weak Bases
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An unknown weak base has an initial concentration of 0.750 M with a pH of 8.03. Calculate its equilibrium base constant.
A
9.35 × 10−9
B
1.53 × 10−12
C
6.54 × 10−3
D
1.07 × 10−6
Verified step by step guidance1
Start by understanding the relationship between pH and pOH. Since pH + pOH = 14, calculate the pOH using the given pH value of 8.03.
Use the pOH to find the hydroxide ion concentration [OH⁻] using the formula: \( \text{pOH} = -\log[\text{OH}^-] \). Rearrange this to find \([\text{OH}^-] = 10^{-\text{pOH}}\).
Recognize that the weak base partially dissociates in water, and the equilibrium concentration of OH⁻ ions is related to the base dissociation constant \( K_b \).
Set up an ICE (Initial, Change, Equilibrium) table to determine the equilibrium concentrations of the base and its dissociation products. Use the initial concentration of the base and the calculated [OH⁻] to find the change in concentration.
Use the expression for the base dissociation constant \( K_b = \frac{[\text{OH}^-][\text{B}^+]}{[\text{B}]} \), where [B] is the equilibrium concentration of the base, to solve for \( K_b \). Substitute the values from the ICE table into this expression.
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