Why is [H3O+] squared when calculating pH of titrations?
HA + H2O --> A- + H3O+
therefore,
Ka = [A-][H3O+] / [HA]
Why does it deduce to:
Ka = [H3O+]^2 / [HA] ?
I can't figure out why H3O+ and A- would have concentrations equal enough to combine the terms. If this was the case why couldn't you assume the same thing with strong acids such as HCl? Wouldn't H3O+ and HCl concentrations also be able to be combined?
Answer:
the equation you have written is not correct for titrating a strong base with a weak acid. neither HA or H2O would be a strong base. If you were looking at a base reaction, it would be more like
HA + H2O --> H2A+ + OH-
if you're looking at a strong acid with a weak base, which is what you have written, then:
HA + H2O ---> A- + H+ (same as H3O+)
you have equimolar amounts of your conjugate base and hydrogen (because they have the same coefficient, 1). since you need to calculate pH, you only need to look at hydrogen. if A- = H+, you can simply substitute H+ for A-.
Becuase when 1 molecule of HA dissociates, it produces 1 H3O+ and 1 A-. Both have the same concentration: [H30+] = [A-]
The same can be said for HCl; both [Cl-] and [H3O+] have the same concentration.
The answers post by the user, for information only, FunQA.com does not guarantee the right.
More Questions and Answers: