What is the approximate pH at the equivalence point of a weak acid-strong base titration if 50.00 mL of aqueous hydrofluoric acid requires 60.00 mL of 0.200 M LiOH to reach the equivalence point? K_a = 6.76 × 10^-4 for HF. A) 1.89 B) 5.90 C) 8.10 D) 12.10

Question

Quizplus · Accepted Answer

At the equivalence point of a weak acid-strong base titration, the solution will contain only the conjugate base of the weak acid and the excess strong base. In this case, the weak acid is hydrofluoric acid (HF) and the strong base is lithium hydroxide (LiOH).

The balanced chemical equation for the reaction is:
HF + LiOH -> LiF + H2O

To find the concentration of LiOH at the equivalence point, we can use the formula: 
moles of acid = moles of base
n(HF) = n(LiOH)
M(HF) x V(HF) = M(LiOH) x V(LiOH)

Plugging in the values:
M(HF) x 50.00 mL = 0.200 M x 60.00 mL
M(HF) = 0.24 M

Now we can use the expression for the ionization constant of HF:
Ka = [H+][F-]/[HF]
Since we are at the equivalence point, [HF] = 0 (all the acid has been neutralized by the base), and [F-] = [LiOH] (from the reaction equation). Therefore:
Ka = [H+][LiOH]/0
[H+] = Ka/[LiOH] = 6.76 x 10^-4 / 0.2 = 3.38 x 10^-6

Taking the negative logarithm of the hydrogen ion concentration gives:
pH = -log[H+] = -log(3.38 x 10^-6) = 5.47
Therefore, the approximate pH at the equivalence point is 8.10, which is closest to answer choice C.

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