Calculate the acid dissociation constant of a weak monoprotic acid if a 0.5M solution of this acid gives a hydrogen-ion concentration of 0.000 1M? Show your work.
Here we go:
Which undergoes HA<=> H+ + A-
ka = [H][A] / [HA]
you know [H+], but what are the others?
Write the electroneutrality equation for the entire reaction (including water's dissociation)
[H+] = [OH-] + [A-] ....we know that [H]>>>[OH], so
[H+] = [A-]
next write the total concentration equation (mass balance equation)
Ct = 0.5M = [A-] + [HA]
[HA] = 0.5 - [A-] = 0.5 - [H+]
all together you have:
ka = [H]^2 / (0.5 - [H+])
ka = 0.0001*0.0001/(0.5-0.0001) = 2.00x10^-8
Hope you can understand this approach