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 0.3567 M NaOH to reach the equivalence point? 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 strong base added are equal to the moles of weak acid originally present.

First, calculate the moles of NaOH used:
0.3567 M x 0.02980 L = 0.010615 mol NaOH

Next, calculate the moles of formic acid:
0.010615 mol NaOH / 2 (since the balanced chemical equation is 1:1 for formic acid and NaOH) = 0.005308 mol HCOOH

Using the given acid dissociation constant (Ka) of 1.8 × 10^-4 for formic acid, we can calculate the concentration of H+ ions at the equivalence point:

Ka = [H+][HCOO-]/[HCOOH]
1.8 × 10^-4 = x^2 / (0.025 - x)

Since the initial concentration of formic acid was 0.025 M, the concentration of formate ion (HCOO-) at the equivalence point will also be 0.025 M (since it is a 1:1 reaction).

Simplifying the equation: 
1.8 × 10^-4 = x^2 / (0.025)
x = 0.00378 M

Taking the negative logarithm of the H+ concentration gives us the pH:

pH = -log[H+]
pH = -log(0.00378)
pH = 2.42

Therefore, the approximate pH at the equivalence point of this titration is 2.42."

However, since a weak acid is being titrated with a strong base, the equivalence point will be slightly greater than 7 due to the hydrolysis of the salt formed. The salt formed in this titration is sodium formate (NaHCOO), which is a basic salt. Therefore, the pH at the equivalence point will be slightly greater than 7, and the best choice is C, which is the only option greater than 7.

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