Frequently Asked Question

The equivalence point of a titration occurs when the moles of acid and base are equal. In this case, the pH at the equivalence point will be slightly basic because HF is a weak acid. To calculate the pH, we need to consider the hydrolysis of the fluoride ion (F-) produced by the reaction. Here's how to approach the calculation:
1. Calculate the moles of HF:
Moles HF = (0.3 M)
(0.025 L) = 0.0075 moles
2. At the equivalence point, moles of HF = moles of NaOH:
Since the concentrations are equal, the volume of NaOH required will also be 25.00 mL.
3. Calculate the concentration of F- at the equivalence point:
The total volume of the solution is now 50.00 mL (25.00 mL HF + 25.00 mL NaOH). The concentration of F- is:
[F-] = 0.0075 moles / 0.050 L = 0.15 M
4. Set up an ICE table for the hydrolysis of F-:
F- + H2O ⇌ HF + OH-
I: 0.15M - 0 0
C: -x - +x +x
E: 0.15-x - x x
5. Use the Kb expression to solve for x (the concentration of OH-):
Kb = Kw / Ka = (1.0 x 10^-14) / (6.6 x 10^-4) = 1.5 x 10^-11
Kb = [HF][OH-] / [F-] = x^2 / (0.15 - x) ≈ x^2 / 0.15
Solve for x (OH-) = 1.5 x 10^-6 M
6. Calculate the pOH and pH:
pOH = -log(1.5 x 10^-6) ≈ 5.82
pH = 14 - pOH ≈ 8.18
Therefore, the pH at the equivalence point is approximately 8.18.