Multiple Choice
What is the pH of a 0.05 M sodium fluoride solution, given that F⁻ has a Kb of 1.59 x 10⁻¹¹?
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 pH of a 7.0 × 10⁻³ M NH₃ solution? (Kb = 1.8 × 10⁻⁵ for NH₃)
A
12.87
B
11.13
C
7.00
D
2.87
Verified step by step guidance1
Identify that NH₃ is a weak base and will partially ionize in water to form NH₄⁺ and OH⁻. The equilibrium expression for this reaction is: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Write the expression for the base dissociation constant (Kb) for NH₃: Kb = [NH₄⁺][OH⁻] / [NH₃].
Set up an ICE (Initial, Change, Equilibrium) table to determine the concentrations of NH₄⁺ and OH⁻ at equilibrium. Initially, [NH₃] = 7.0 × 10⁻³ M, and [NH₄⁺] = [OH⁻] = 0 M.
Assume that the change in concentration of NH₃ is 'x', so at equilibrium, [NH₃] = 7.0 × 10⁻³ - x, [NH₄⁺] = x, and [OH⁻] = x. Substitute these into the Kb expression: 1.8 × 10⁻⁵ = (x)(x) / (7.0 × 10⁻³ - x).
Solve the quadratic equation for 'x' to find [OH⁻], then calculate the pOH using the formula pOH = -log[OH⁻]. Finally, convert pOH to pH using the relation pH + pOH = 14.
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