Multiple Choice
What is the pH of a solution containing 400 mg of caffeine with a pKb of 10.4, if the solution has a 3.44% mass concentration and a density of 1.01 g/mL?
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17. Acid and Base Equilibrium
pH of Weak Bases
Multiple Choice
What is the theoretical pH of a 0.1 M solution of sodium acetate (NaC2H3O2), a weak base?
A
9.25
B
5.13
C
7.00
D
8.87
Verified step by step guidance1
Identify that sodium acetate (NaC2H3O2) is a salt of a weak acid (acetic acid, CH3COOH) and a strong base (NaOH). In water, it will hydrolyze to form acetate ions (C2H3O2^-) which can accept protons, making the solution basic.
Write the hydrolysis equation for the acetate ion: C2H3O2^- + H2O ⇌ CH3COOH + OH^-. This shows that acetate ions react with water to form acetic acid and hydroxide ions, increasing the pH.
Use the equilibrium expression for the hydrolysis reaction: Kb = [CH3COOH][OH^-] / [C2H3O2^-]. The Kb can be found using the relationship Kb = Kw / Ka, where Kw is the ion-product constant of water (1.0 x 10^-14) and Ka is the acid dissociation constant for acetic acid (1.8 x 10^-5).
Calculate the concentration of hydroxide ions [OH^-] using the expression derived from the Kb: [OH^-] = sqrt(Kb * [C2H3O2^-]). Substitute the known values to find [OH^-].
Determine the pH by first calculating the pOH from the concentration of hydroxide ions: pOH = -log[OH^-]. Then, use the relationship pH + pOH = 14 to find the pH of the solution.
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