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 M 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

The balanced equation for the reaction is:
HF(aq) + NaOH(aq) → NaF(aq) + H2O(l)

From the stoichiometry of the reaction, we can calculate the moles of NaOH used:

moles NaOH = Molarity x Volume (in liters)
moles NaOH = 0.400 M x 0.03000 L
moles NaOH = 0.01200 mol

Since NaOH and HF react in a 1:1 ratio, we know that 0.01200 mol of HF was also present in the solution. We can use this information to find the initial concentration of HF:

Initial moles HF = Final moles HF
Initial concentration of HF x Initial volume of HF = Final concentration of HF x Final volume of HF

Solving for the initial concentration of HF gives:
Initial concentration of HF = (Final concentration of HF x Final volume of HF) / Initial volume of HF
Initial concentration of HF = (0.01200 mol / 0.02500 L) = 0.480 M

The pH at the equivalence point of the titration can be calculated using the Ka value for HF:

Ka = [H+][F-] / [HF]
6.76 × 10-4 = x2 / 0.480
x = 1.74 x 10-3 M

pH = -log[H+]
pH = -log(1.74 x 10-3)
pH = 2.76

At the equivalence point, all of the HF has reacted with NaOH to form NaF and H2O. The resulting solution is the salt NaF and water, which is neutral. Therefore, the pH at the equivalence point will be 7. This value is higher than the calculated pH of 2.76 because of the buffer effect of the HF/F- system before the equivalence point.

Since the question asks for the approximate pH at the equivalence point, we round our calculated value of 2.76 to the nearest whole number, which is 3. Therefore, the best choice is C, 8.25, which is the option closest to 7 (the pH at the equivalence point), but still acknowledges the somewhat acidic pH due to the HF/F- buffer system.

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