[Katy543]

I can't seem to figure out how to do this calculation:

Calculate the ratio of dissolved benzoic acid to benzoate ion that will exist in solution at equilibrium at a pH of 2.  Acid equilibrium constant for benzoic acid is 6.46 x 10^-5.

the reaction is =>  benzoate ion + hydronium yeilds benzoic acid + water

If anyone can help, I will really appreciate it.

Thanks.

[Yggdrasil]

Do you know the definition of the acid equilibrium constant in terms of the concentrations of benzoate, benzoic acid, and hydronium ion?

[Katy543]

Ka= [Benzoic Acid]/[hydronium]*[benzoate ion]

is that what you mean???
I still dont understand how you would find the ratio

[Yggdrasil]

Isn't Ka = [benzoate][hydronium]/[benzoic acid]?

Can you rearrange this equation to find an expression for [benzoic acid]/[benzoate] in terms of Ka and [hydronium]?

Alternatively, to solve this problem, you can use the Henderson-Hasselbalch equation.

[Katy543]

Yea you can, and you get [hydronium]*Ka= [BA]/

i calculated  it and the answer equaled out to be 0.00646....thats a small number, but i guess from the equation it makes sense.

Thanks for the help

[Yggdrasil]

Ka = [benzoate][hydronium]/[benzoic acid]

So
[benzoic acid]/[benzoate] = [hydronium]/Ka

This number turns out to be a large number which makes sense because in a very acidic environment, you should have more of the protonated form of the acid than the deprotonated form.