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Understanding dissociation constants

by sodium.dioxid
Tags: constants, dissociation
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sodium.dioxid
#1
May6-12, 11:15 PM
P: 51
Ka = [H+][A-]/[HA]

Is the term "[HA]" referring to its initial concentration or its equilibrium concentration?
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Borek
#2
May7-12, 01:57 AM
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All concentrations are equilibrium concentrations. Initial concentration of acid (sometimes called formal or analytical) is [HA]+[A-].
sodium.dioxid
#3
May7-12, 05:52 PM
P: 51
OK, another question: why is it necessary to have both [H+] and [A-] on the numerator? It seems like we could have done just as fine using the definition "Ka=[A-]/[HA]" or "Ka=[H+]/[HA]" instead of "Ka=[H+][A-]/[HA]". Is this conventional or am I missing something?

Borek
#4
May8-12, 01:48 AM
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Understanding dissociation constants

As both H+ and A- are products of the reaction changing concentration of either one shifts the equilibrium, so you need both.


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