Acids and Bases general q

[Sammy1024]

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If water is being added to a solution, how would I go about calculating a pH change?

Also, Is there a way to calculate pH given only the pka? Like they say something like pKa is 8, what pH do you expect with a weak acid of 0.1 M. Would i use pH = 1/2 pka + log base/acid even though I only have the concentration of the weak acid?

 

[Teleologist]

pH is the negative log of the hydronium ion concentration, so if you add water, you're diluting the solution. Calculate the new concentration of hydronium ion and take its negative log to find pH.

And no, you wouldn't use the Henderson-Hasselbach equation because that's not a buffer system you are speaking about. Use the pKa to derive the Ka and remember what the Ka of a weak acid is equal to (consider the ionization equilibrium of a weak acid).

 

[Sammy1024]

I meant to write pH = 1/2 pka + 1/2 log (b/a) I think I read in TBR that it was a shortcut for solving for pH

 

[labqi]

That's for a weak acid I believe. Add pure water to a buffer doesn't change the ph