Multiple Choice
Calculate the pH of a 0.0095 M solution of acetic acid, CH3COOH, given that the acid dissociation constant (Ka) is 1.8 x 10^-5.
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17. Acid and Base Equilibrium
pH of Weak Acids
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Calculate the pH of a 0.300 M NaHCOO solution, given that the Ka for HCOOH is 1.8 x 10^-4.
A
pH = 6.50
B
pH = 8.24
C
pH = 7.00
D
pH = 5.76
Verified step by step guidance1
Identify that NaHCOO is the salt of a weak acid (HCOOH) and a strong base (NaOH), which means it will hydrolyze in water to form a basic solution.
Write the hydrolysis equation for the formate ion (HCOO-): HCOO- + H2O ⇌ HCOOH + OH-.
Use the relationship between Ka and Kb to find the Kb for the formate ion: Kb = Kw / Ka, where Kw is the ion-product constant of water (1.0 x 10-14 at 25°C).
Set up the expression for Kb: Kb = [HCOOH][OH-] / [HCOO-]. Assume that the change in concentration of HCOO- due to hydrolysis is small, so [HCOO-] ≈ 0.300 M.
Solve for [OH-] using the Kb expression, then calculate the pOH of the solution. Finally, use the relationship pH + pOH = 14 to find the pH.
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