Question

A weak acid HA with a pKa = 2.00 is added to a solution with a fix pH = 8.00. Estimate the percent dissociation of the acid under such conditions

Answer 1

HA is acid and its dissociated form is A-     ( conjugate base)

we have Henderon eq for pH as   

pH = pka + log [ conjugate base] /[acid]

8 = 2 + log [A-]/[HA]

[A-] /[HA] = 10^6

Thus per we have 1HA molecule per 10^6 A- molecules. All A- is formed from HA , hence initial HA = 1+10^6 = 1000001

Hence % of [A-] = 100 x [A-] /[HA]    = 100 x 10^6 / ( 1000001)

    = 99.9999 %

Thus dissociation percentage is nearly 100