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A monoprotic weak acid, HA, dissociates in water according to the reactionHA(aq)+H2O(l)↽−−⇀H3O+(aq)+A−(aq)The equilibrium concentrations of the reactants and products are [HA]=0.140 M, [H3O+]=4.00×10−4 M, and [A−]=4.00×10−4 M. Calculate the ????a value for the acid HA.

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JannetPalos

Answer:

[tex]K_{a}=\frac{(0.0004)^{2}}{0.140}=1.14\times 10^{-6}[/tex]

Explanation:

Equilibrium constant of this acid dissociation equilibrium is expressed as-

[tex]K_{a}=\frac{[H_{3}O^{+}][A^{-}]}{[HA]}[/tex]

species inside third bracket represent equilibrium concentrations.

Here concentration of [tex]H_{2}O[/tex] is taken as unity as concentrations of pure liquids and solids remain constant.

Here, [HA]=0.140M,[tex][H_{3}O^{+}]=0.0004M[/tex] and [tex][A^{-}]=0.0004M[/tex]

So, [tex]K_{a}=\frac{(0.0004)^{2}}{0.140}=1.14\times 10^{-6}[/tex]

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