Question

Find Pka value and equivalence point for each :)

Titration of Acetic Acid with Sodium Hydroxide PH 0 0.1 0.2 0.3 04 Volume NaOH added 0.5 0.6

Answer 1

For a weak acid, the equivalence point is at the center of the steep part of the titration curve. This is where the number of moles of acid titrated = the number of moles of base. At halfway to the equivalence point, the concentration of acid presesnt is equal to the concentration of the acid converted to its conjugate base. Thus, at this point pH = pKa.

According to these observations,

For Fig 1, pH at equivalence point is around 9 and pKa will be around 4.5.

For Fig 2, selenious acid is a polyprotic acid. It has two equivalent points. pH at first equivalence point is around 3.5 and at the second equivalence point pH is around 9. Thus, pKa1 = 2.3 and pKa2 is around 7.

For Fig 3, pH at equivalence point is around 8 and pKa will be around 4.