What is the approximate pH at the equivalence point of a weak acid-strong base titration if 25 mL of aqueous formic acid (HCOOH) requires 29.80 mL of 0. 3567 mol L^-1 NaOH? K_a =1.8 × 10^-4 for formic acid. A) 2. 06 B) 5. 48 C) 8. 52 D) 11.94

Question

Quizplus · Accepted Answer

At the equivalence point of a weak acid-strong base titration, the moles of the strong base added are equal to the moles of weak acid originally present.

First, we need to determine the moles of sodium hydroxide used in the titration:

0.3567 mol/L x 0.02980 L = 0.01062556 mol NaOH

Since the reaction between formic acid and sodium hydroxide is a 1:1 stoichiometry, we know that 0.01062556 moles of formic acid were present in the initial solution.

Now we can use the equilibrium expression for formic acid to determine the pH at the equivalence point:

Ka = [H+][HCOO-]/[HCOOH]

Ka = 1.8 x 10^-4 (given)

At the equivalence point, [HCOOH] = [HCOO-] (since they are both weak acid and its conjugate base) and [H+] = [OH-] (since the solution is neutral)

Therefore, Ka = [H+]^2/[HCOOH]

1.8 x 10^-4 = x^2/0.01062556

x = 0.004109 M

pH = -log[H+] = -log(0.004109) = 2.57

Since the pH is less than 7, we know that the solution is acidic. The best choice among the answer options is C) 8.52, since it is the only one greater than 7 and therefore incorrect.

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