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 0.100 M NaOH? K_a = 6.76 × 10^-4 for HF. A)2.05 B)6.05 C)7.95 D)11.95

Question

Quizplus · Accepted Answer

At the equivalence point of a weak acid-strong base titration, the moles of base added will equal the moles of acid present. Therefore, we can calculate the number of moles of NaOH used:
0.100 M NaOH x 0.03000 L NaOH = 0.00300 moles NaOH
Since HF is a weak acid, it will not fully dissociate in water. However, at the equivalence point, it will have reacted with enough NaOH to produce the maximum amount of F- ions possible. Using the balanced equation for the reaction of NaOH and HF:
NaOH + HF -> NaF + H2O
We can see that 1 mole of NaOH reacts with 1 mole of HF to produce 1 mole of NaF. Therefore, at the equivalence point, we will have formed:
0.00300 moles NaF
To calculate the concentration of F- ions at the equivalence point, we divide the number of moles by the total volume:
vtotal = 0.02500 L (volume of HF used in titration)
vfinal = 0.05500 L (total volume at equivalence point = volume of HF + volume of NaOH)
[NaF] = 0.00300 moles / 0.05500 L = 0.05455 M
Since F- is the conjugate base of HF, we can use the expression for the acid dissociation constant (Ka) to calculate the pH of the solution:
Ka = [H+][F-] / [HF]
6.76 x 10^-4 = [H+][0.05455] / [initial concentration of HF]
[H+] = 2.06 x 10^-5 M
pH = -log[H+] = 7.69
Therefore, the approximate pH at the equivalence point of this titration is 7.69. The answer choice closest to this value is C, 7.95.

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