[Cassowary]

I've been trying to answer a problem:


The pH of an aqueous solution obtained by dissolving 0.49 g of o-nitrobenzoic acid in 707 mL of pure H2O is ____."

The answer is supposed to be 2.54, but I can't seem to get that answer. The values I have for the molecular weight of o-nitrobenzoic is 167.12 g/mol, the pKa as 2.19. I've been using the Henderson–Hasselbalch equation.

Can anyone tell me what I'm doing wrong? Or is the supposed answer wrong?

[Borek]

HH equation doesn't make much sense for this question. There is no buffer here, just a solution of the weak acid.

2.54 doesn't look correct to me. I got about 0.3 less.

[Cassowary]

Blah, didn't mean HH, sorry, I meant whatever this process is: http://en.wikipedia.org/wiki/Weak_acid

I end up getting 1.21...

pKa = 2.19 -> Ka = 0.006456

x is [H+]

Ka = (x^2)/(0.004147-x) ->

x^2 + (Ka)(x) - (Ka)(0.004147) -> quadratic

And I end up with 1.21.

[Borek]

Ka = (x^2)/(0.004147-x) ->

x^2 + (Ka)(x) - (Ka)(0.004147) -> quadratic

And I end up with 1.21.

Check your math.

Sorry, I was wrong yesterday, 2.54 seems to be the correct answer.