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(aq)? 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 the acid are equal to the moles of the base added.

Moles of NaOH = 0.4 mol/L × 0.030 L = 0.012 mol

From the balanced chemical equation: 
HF + NaOH → NaF + H2O

It can be seen that 1 mole of NaOH reacts with 1 mole of HF. Therefore, 
Moles of HF = 0.012 mol

The initial concentration of HF can be calculated using its volume and the given K value:

K = [F-][H+] / [HF]

6.76 × 10^-4 = x^2 / (0.025 L × 0.05 L)

x = [H+] = 0.0821

pH = -log[H+] = 8.25

Therefore, the approximate pH at the equivalence point of the titration is 8.25, which is answer choice C.

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