A weak acid is titrated with a strong base to the equivalence point.The pH of the resulting solution is found to be 9.18.The pK_a of the acid is A) 9.18. B) 7.00. C) 4.59. D) 2.50. E) 6.28.

Question

Quizplus · Accepted Answer

At the equivalence point, the moles of acid and base are equal and hence the resulting solution will be a salt solution of the acid and base.
Let HA be the weak acid.
HA + NaOH → NaA + H2O
At the equivalence point, all the HA is converted to NaA.
So, [NaA] = [HA]initial - [OH-] = C - C = 0
This implies that NaA is completely hydrolyzed and the solution is basic in nature.
NaA + H2O ⇌ HA + Na+ + OH-
[OH-] = [HA]
pOH = 9.18
pH + pOH = 14
pH = 4.82
So, [HA] = 10^(-4.82)
Ka = [H+][A-]/[HA] = Kw/[OH-] = 1.0 x 10^-14/10^(-9.18) = 1.7 x 10^-5
pKa = 4.59.

A Weak Acid Is Titrated with a Strong Base to the Equivalence