Question

A 0.22 −mol sample of a weak acid with an unknown pKa was combined with 12.0 mL of 3.20 M KOH and the resulting solution was diluted to 1.500 L. The measured pH of the solution was 3.90. What is the pka of the weak acid?

Answer 1

pka of the weak acid = 4.64

Step-by-step explanation:

pH of solution is calculated by using Henderson - Hasselbalch equation

`pH=pKa+log(([A^(-) ])/([HA]) )` --------------------(i)

`Moles of KOH =(12)/(1000) X3.2=0.0384`

HA + KOH ⇄ HK + H₂O

Initial 0.25 0.0384 0 0

Equm. 0.2116 0 0.0384

From equation (i)

`pKa = pH - log(([A^(-) ])/([HA]) )\\\\ =3.9 - log((0.0384)/(0.2116) )\\\\= 3.9 - log (0.18)\\\\= 3.9+0.74\\\\=4.64`