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?
K_a = 1.8 × 10^-4 for formic acid.

Question

Quizplus · Accepted Answer

At the equivalence point, moles of NaOH added = moles of formic acid present in the solution.

Moles of NaOH = (0.3567 mol/L) x (0.02980 L) = 0.01063 mol

Moles of formic acid = Moles of NaOH = 0.01063 mol

Concentration of formic acid = moles/volume = 0.01063 mol / 0.025 L = 0.4252 M

pKa for formic acid = 1.8 x 10^-4

pH at equivalence point = pKa + log (base/acid)

The base/acid ratio can be calculated using the Henderson-Hasselbalch equation:

pH = pKa + log (base/acid) = pKa + log ( [NaOH]/[formic acid]) = pKa + log (0.3567 M / 0.4252 M)

pH = 8.52

Therefore, the pH at the equivalence point of the weak acid-strong base titration is approximately 8.52.

The answer is C.

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