[Rutherford]

Is there a suitable way to calculate the pH of a weak acid solution whose concentration is very small (water dissociation makes the problem)? For example: Calculate the pH of an acetic acid solution whose c=1*10^-7M (Ka=1.8*10^-5). How would I do this?

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-strong-acid-base

[Mr-E]

I am still new to learning chemistry so I hope I didn't misunderstand the question completely or make any mistakes. Let me know

Example: Ethanoic Acid, Ka = .000017
Concentration of Ethanoic Acid we were given = .0000001
Write the equation first: CH3COOH + H2O ---> H3O+ + CH3COO-
Then plug this into the equilibrium formula. Products / Reactants (not water) = Ka

[H3O+][CH3COO-] / .0000001 = 000017
As the products are in 1 : 1 ratio you can say [H3O+]Squared / .0000001 = 000017
Transpose the equation:

.0000001 * .000017 = [H3O+]Squared
=.0000000000017
since this is squared and we only want the [H3O+] we square root it.
Square root of .0000000000017 = .000001304

pH = -log [H3O+]
pH = -log .000001304
pH = 5.885

[Rutherford]

http://www.chembuddy.com/?left=pH-calculation&right=pH-strong-acid-base

I know that, but I was asking for a weak acid/base solution, which I found under the strong acid/base title (http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base). There, between the steps 8.4 and 8.5 is neglected what I was asking here, so it wasn't very helpful.

Mr-E, the problem is that water dissociation takes an important role at low concentrations of acids and bases.

[Mr-E]

I understand. Well GL

[Borek]

Sorry, I was answering ion haste and missed the weak acid part.

In general your problem requires solving third degree polynomial. See general equation 6.11 here: http://www.chembuddy.com/?left=pH-calculation&right=pH-acid-base-solution

[Rutherford]

Thanks. I was looking for that.