What is the approximate pH at the equivalence point of a weak acid-strong base titration if 25 mL of aqueous hydrofluoric acid requires 30.00 mL of 0. 400 mol L^-1 NaOH? K_a = 6.76 × 10^-4 for HF. A) 1.74 B) 5.75 C) 8.25 D) 12.26

Question

Quizplus · Accepted Answer

At the equivalence point of a weak acid-strong base titration, the moles of strong base added equal the moles of weak acid originally present. To find the moles of HF present, we can use the concentration and volume:

moles HF = (conc. HF) x (volume HF) = (unknown) x 0.025 L

Next, we can use the balanced chemical equation for the reaction between HF and NaOH:

HF + NaOH -> NaF + H2O

Since the reaction is 1:1, the moles of NaOH added at the equivalence point equal the moles of HF originally present. We can use this information to find the concentration of NaOH:

moles NaOH = (conc. NaOH) x (volume NaOH) = (0.4 mol/L) x 0.03 L
moles NaOH = moles HF = (unknown) x 0.025 L

Solving for the concentration of HF:

0.4 mol/L x 0.03 L = (unknown) x 0.025 L
conc. HF = 0.48 M

Now we can use the Ka expression for HF to calculate the pH at the equivalence point:

Ka = ([H+][F-])/[HF]
6.76 x 10^-4 = (x^2)/(0.48-x)

Since the initial concentration of HF is much larger than x, we can assume that (0.48 - x) ≈ 0.48:

6.76 x 10^-4 = (x^2)/(0.48)
x^2 = 3.2448 x 10^-4
x = 0.018

pH = -log[H+] = -log(0.018) = 1.74

Therefore, the answer is C) 8.25.

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