[justsavory]

1Benzoic acid (0.500 g, pKa = 4.204) was dissolved in 100 mL of an aqueous buffer and the pH was adjusted to 7.0. The aqueous solution (100 mL) was extracted with one 100-mL portion of diethyl ether. The ether layer was separated and the solvent was evaporated to yield 3.03 mg of benzoic acid. Calculate the partition coefficient of benzoic acid between ether and water.

I have no idea how to find the concentration of the benzoic acid ([HA]_aqueous) when it is at equilibrium in the aqueous buffer at pH 7.0 

[Borek]

Look at the dissociation constant definition - what do you know? What you are looking for? Hint: you are told what is the total concentration, that is sum of HA of A^-.

[Hunter2]

@:Borek

Even you erased my post. I want to asked what was wrong with my answer. If I have two Phases one with Ether one with Water. In the water phase 0,5 g benzoec acid was dissolved . After shaking and seperation of the ether. 3 mg of the acid was found in ether.
Why does it matter what is the dissozation rate in water. Before it was 0,5 g after treatment it has to be 497 mg.  Or do they asked of the amount of undissociated acid?

[justsavory]

@:Borek

Even you erased my post. I want to asked what was wrong with my answer. If I have two Phases one with Ether one with Water. In the water phase 0,5 g benzoec acid was dissolved . After shaking and seperation of the ether. 3 mg of the acid was found in ether.
Why does it matter what is the dissozation rate in water. Before it was 0,5 g after treatment it has to be 497 mg.  Or do they asked of the amount of undissociated acid?

The partition coefficient equation is K_pc = [HA]_ether/[HA]_aqueous. It doesn't factor in the dissociated acid (conjugate base) because it is insoluble in the diethyl ether. So to answer your question in order to calculate the correct answer you do have to find the amount of undissociated acid.

[Borek]

I want to asked what was wrong with my answer.

You should don't give final answers, that's all. After being a member for almost 4 years you should know the rules.