Multiple Choice
What is the percent ionization of a 0.16 M benzoic acid solution in a solution containing 0.28 M sodium benzoate, given that the Ka of benzoic acid is 6.3 x 10^-5?
17. Acid and Base Equilibrium
pH of Weak Acids
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Determine the pH of a 0.036 M solution of formic acid (HCO2H), given that the acid dissociation constant (Ka) is 1.8 x 10^-4. The dissociation reaction is: HCO2H (aq) ⇌ H+ (aq) + HCO2- (aq).
A
pH = 2.37
B
pH = 4.12
C
pH = 1.85
D
pH = 3.45
Verified step by step guidance1
Identify the initial concentration of formic acid (HCO2H), which is given as 0.036 M.
Write the expression for the acid dissociation constant (Ka) for formic acid: \( K_a = \frac{[H^+][HCO_2^-]}{[HCO_2H]} \).
Assume that the concentration of H+ ions produced is 'x'. At equilibrium, [H+] = [HCO2-] = x and [HCO2H] = 0.036 - x.
Substitute these equilibrium concentrations into the Ka expression: \( 1.8 \times 10^{-4} = \frac{x^2}{0.036 - x} \).
Assume x is small compared to 0.036, so 0.036 - x ≈ 0.036. Solve for x using \( x^2 = 1.8 \times 10^{-4} \times 0.036 \), then calculate pH as \( pH = -\log[H^+] \).
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