Chemistry
posted by Chemistry Confused .
Two Concept Questions for you:
1) Why is the concentration of a solution such as NaH2PO4 ignored when calculating the pH given Ka1, Ka2, and K3 values?
2) Given Pka1=3.13, Pka2=4.76, and Pka3 = 6.40, calculate the pH of lemon juice which is about 5% citric acid.
I know I have to just go pH= (1/2) pka1,
but why is it just (1/2) of Pka1? Where did the (1/2) come from?

1. If you derive the formula for (H^+) at the (say) first equivalence point, the concn of the salt present cancels and one is left with (H^+) = sqrt(k1k2) which is 1/2(pk1 + pk2) = pH.
2. Citric acid is so highly buffered between the beginning and the final (only) practical equivalence point, that the HendersonHasselbalch equation of
pH = pK + log(base/acid) is essentially pK and log (base/acid) is log 1 = 0
Here is a site that talks about that but only a little.
http://www.titrations.info/acidbasetitrationpolyproticsandmixtures 
thanks. that helped!!