What is the approximate pH at the equivalence point of a weak acid-strong base titration if 25 mL of aqueous formic acid requires 29.80 mL of 0.1567 M NaOH? K_a =1.8 × 10^-4 for formic acid. A) 2.24 B) 5.66 C) 8.34 D) 11.76

Question

Quizplus · Accepted Answer

At the equivalence point of a weak acid-strong base titration, the moles of strong base added equals the moles of weak acid present in the solution. Therefore, we can use the following equation to calculate the concentration of formic acid:

(0.02980 L)(0.1567 mol/L) = (0.025 L)(x mol/L)

x = 0.184 M

Since formic acid is a weak acid, it will not fully dissociate in water. However, at the equivalence point, all of the weak acid has been neutralized by the strong base, resulting in a solution of its conjugate base, formate ion.

We can use the equilibrium expression for the dissociation of formic acid to calculate the pH of the solution at the equivalence point:

formic acid + water ⇌ formate ion + hydronium ion

Ka = ([H3O+][HCOO-])/[HCOOH]

1.8 × 10^-4 = (x^2)/(0.184 - x)

Solving for x, we get:

[H3O+] = 1.70 × 10^-6 M

pH = -log[H3O+] = 8.77

Therefore, the approximate pH at the equivalence point of this titration is 8.34 (closest answer choice is C).

What Is the Approximate PH at the Equivalence Point of a Weak