Unformatted text preview: KA1 / KA2 = 7.5x10-3 / 6.0x10-8
= 1.25 x 105 KA2 / KA3 = 6.0x10-8 / 4.8x10-13
= 1.25 x 105 This indicates that it is possible to
treat each step separately during
a titration. Just because you have a material where KA1/KA2
is not greater than 1000 does not mean it
can’t be titrated. We’ll now review the calculations involved for
the titration of a ‘well behaved’ diprotic acid. It is still possible to determine equilibrium
concentrations at any point during a titration. This really is nothing more that doing two
separate sets of calculations. However, when KA1/KA2 < 1000, you don’t get
a sharp ‘first’ endpoint. You can
Titrate to the ‘final endpoint’
Titrate to a form where Kx/KY > 1000. Triprotic acids (or higher) simply amount to
repeating the process as many times as
necessary. Construct a curve for the titration of 25.00 ml of
0.1000 M sodium carbonate (Na2CO3) with
0.1000 M HCl.
KB1 = 2.00 x 10-4 = [ OH- ] [ CO32- ]
[HCO3- ] KB2 = 2.51 x 10-8 = [ OH ] [HCO3 ] [ H2CO3 ] KB1 / KB2 = 8.00 x
so our base is ‘well behaved’ and we can treat
each equilibrium separately.
103 KB1 = 2.00 x 10-4 = [ OH- ] [ CO32- ]
[HCO3- ] [ OH- ] = [ CO32- ]
[ HCO3- ] = 0.1000M - [ CO32- ] ...
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- Fall '08
- Acids, Bicarbonate, ‘first’ endpoint.