[Edher]

Saludos,

     I am confident in the metho I am using to solve this problem, however, I still can't get the right answer:

What are [H₃O], [OH^- ], pH, and pOH of 0.55 M HCHO₂?

I set up my equation,

HCHO₂ + H₂O <> H₃O⁺ + CHO₂^-
I   0.55M                                              0                              0

C   -x                                                       +x                       +x

E   0.55 - x                                             x                        x


Ka = 1.8e-4   (according to my book)

This is what I did:

          1.8e-4 = x²
                     0.55-x


I got x = 9.86e-3

which according to me should be the [H₃O ] but according to my book the answer is 0.073M.

What am I doing wrong?

[jubba]

seems right

your pka value might be wrong

[Borek]

3.75

Looks to me you have use Ka of acetic acid, not formic.

[Edher]

According to my book, and charts I looked up online, the Ka of formic acid (  HCHO₂ ) is 1.8 x 10^-4 . Which is what I used to solve the problem but I still didn't get the same answer as in the book. They got [ H₃O⁺ ] = 0.073 M. But I keep getting 9.94 x 10^-3 M.

Any suggestions?

Thank You,
Edher

[Borek]

Sorry, my mistake 10^-4 is formic, 10^-5 is acetic, 1.8 being identical in both cases.

Thrash the book. Your answer is correct - pH=2.01, [H⁺ ] = 9.9x10^-3M. Checked with BATE to avoid further mistakes

[Edher]

Great, thanks a lot Borek. Believe me, this is not the first time that I've been tempted to follow your advice of trashing the book. I didn't know we had a program to calculate this problems, and here I am unnecessarily toiling. Too bad I can't use this website on the test.

Edher