Show that benzoic acid is 50% ionized at pH 4.2. pKa is also 4.2?

I'm thinking that henderson Hasslebach equation is probably used but I am not sure.

Answer:
All you need is the equilibrium equation for benzoic acid.

ka=[benzoate][H+]/[benzoic acid]
because pH = pKa; [H+] = ka

so these two terms cancel and you are left with

1=[benzoate]/[benzioc acid] or

[benzoate] = [benzoic acid]
at pH 4.2
because the concentrations are equal benzoic acid is 50% ionized

Plugging numbers in the henderson hasselbach equation would give you the same result

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