Multiple Choice
If 0.365 g of sodium carbonate (Na2CO3) is dissolved in 1.5 L of water, what is the pH of the solution? (Kb for CO3^2- = 1.8 x 10^-4)
17. Acid and Base Equilibrium
pH of Weak Bases
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What is the pH of a 0.145 M solution of (CH3)3N, given that the Kb of (CH3)3N is 6.4 × 10⁻⁵?
A
12.45
B
7.00
C
11.12
D
2.88
Verified step by step guidance1
Identify the base dissociation reaction for (CH3)3N in water: (CH3)3N + H2O ⇌ (CH3)3NH⁺ + OH⁻.
Write the expression for the base dissociation constant (Kb): Kb = [ (CH3)3NH⁺ ][ OH⁻ ] / [ (CH3)3N ].
Set up an ICE (Initial, Change, Equilibrium) table to determine the concentrations of species at equilibrium. Initially, [ (CH3)3N ] = 0.145 M, and [ (CH3)3NH⁺ ] = [ OH⁻ ] = 0 M.
Assume that x is the change in concentration for (CH3)3NH⁺ and OH⁻ at equilibrium. Therefore, [ (CH3)3NH⁺ ] = x, [ OH⁻ ] = x, and [ (CH3)3N ] = 0.145 - x.
Substitute the equilibrium concentrations into the Kb expression and solve for x: 6.4 × 10⁻⁵ = (x)(x) / (0.145 - x). Once x is found, calculate pOH = -log[OH⁻] and then find pH using pH = 14 - pOH.
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