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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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judymarucha judymarucha
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11 years ago
Just having a bit of trouble getting my head around it, because I know that pH = pKa halfway to the equivalence point.. Both tell us the strength of the acid?? I thought I understood it but now I'm just getting confused. Any help will be greatly appreciated!
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#1
11 years ago
pH = -log([H+])
pKa = -log(Ka)
HA Leftwards Arrow> H^+1 + A^-1
Ka = [H+][A-] / [HA]

Ka is a measure of the extent that HA dissociates into H+ and A-. For a strong acid, Ka is infinitely large (i.e. 100% dissociation).

For a weak acid we can solve the Ka equation for [H+ getting-
[H+] = Ka * [HA]/[A-]
take the negative log of both sides to get-
-log([H+]) = -log(Ka) - log([HA]/[A-])
pH = pKa - log([Ha]/[A-]) = pKa + log([A-]/[HA])

pH is a measure of the hydrogen ion concentration while pKa is a measure of the degree of dissociation of the acid.
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