Multiple Choice
What is the ratio of HCO₃⁻ to H₂CO₃ in blood with a pH of 7.4?
17. Acid and Base Equilibrium
pH of Weak Acids
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Calculate the pH of a 0.250 M CH3NH3I solution, given that the Kb for CH3NH2 is 4.4 x 10^-4.
A
9.05
B
8.72
C
4.95
D
5.28
Verified step by step guidance1
Identify the species in the solution: CH3NH3I dissociates into CH3NH3+ and I-. CH3NH3+ is the conjugate acid of CH3NH2.
Write the equilibrium expression for the dissociation of CH3NH3+ in water: CH3NH3+ + H2O ⇌ CH3NH2 + H3O+. The equilibrium constant for this reaction is Ka.
Calculate Ka using the relationship between Ka and Kb: Ka = Kw / Kb, where Kw is the ion product of water (1.0 x 10^-14 at 25°C). Substitute Kb = 4.4 x 10^-4 into the equation to find Ka.
Set up an ICE table (Initial, Change, Equilibrium) for the dissociation of CH3NH3+ to find the concentration of H3O+ at equilibrium. Assume initial concentration of CH3NH3+ is 0.250 M and initial concentration of H3O+ is 0.
Use the equilibrium concentration of H3O+ to calculate the pH: pH = -log[H3O+].
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