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.

First we create a balanced equation

HA H (+) + A (-)

Then, we set up an ICE table

HA H (+) + A (-)

[]i 0.5M 0M 0M

Δ[] - x + x + x

[]f 0.5M-x 0.0001M 0.0001M

From this, we can say that x = 0.0001.

Next, we set up the equation

Ka = [H+][A-]/[HA]

Ka = [0.0001][0.0001]/[0.5-0.0001]

Ka = 2x10^-8