Dissociation constants of weak acids

[Pat Says]

We have an unkown acid (we think its acetic) and a rough idea of the procedure goes like so... dissolve acid into 50ml of water, take 25ml and then titrate until the indicator turns pink... then add the rest of the 25 ml (untitrated) to the titrated acid. then take the pH of the new solution. Anyways, according to our lab, pKa is supposed to equal pH. I cannot figure out at all why pKa = pH. That would mean Ka = H+. One of our questions is to answer why pH = pKa and I don't think it should. If anyone has any idea I would appreciate it very much.

 

(haven't posted in awhile... been really busy )

 

Edit: my bad wrong forum

[ecoli]

pka does not equal pH. On your resulting titration curve, the pH is equal to the pka at the halfway point.

 

I did a similiar lab... this may help you out

http://www.ic.sunysb.edu/Class/che133/lectures/calc010.html

http://www.ic.sunysb.edu/Class/che133/lectures/05Post.pdf

[Pat Says]

That's what I thought... the lab is not making sense at all. Thanks for the links though, maybe I can argue it a little with my chemistry teacher :\.

Would Ka be twice the H+ concentration though?

[ecoli]

HA <--> H+ + A-

 

[math]Ka = \frac{[H^{+}][A^{-}]} {[HA]}[/math]

 

Where the stuff inside the brackets indicates the concentrations at equilibrium (not initial concentrations.)

[Ilja]

the pKa doesnt really equal the pH but you can caclutate using these formulas

 

pH = -lg c(H+) ; ka = -lgpKa

 

H+ = (ka*ca)^1/2 ca=concentration acid