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?
17. Acid and Base Equilibrium
pH of Weak Bases
Struggling with General Chemistry?
Join thousands of students who trust us to help them ace their exams!Watch the first videoMultiple 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.
0:51mWatch next
Master ICE Charts of Weak Bases with a bite sized video explanation from Jules
Start learningRelated Videos
Related Practice
pH of Weak Bases practice set
Problem sets built by lead tutorsExpert video explanations