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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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fishtaco fishtaco
wrote...
12 years ago
if a question asks rank the acids strongest to weakest... and it only gives the pKa... how would i determine which acid is stronger?

Examples:
HF (pKa=15)
HNO2 (pKa=7.5)
NH4+ (pka=10.5)
NH3 (pKa= 41)
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buffalo Author
wrote...
#1
12 years ago
For a weak acid:
HA  ----> H+ + A-
Ka = [H+][A-] / [HA]
The larger the Ka value the stronger the acid, because the products are in the numerator of the Ka expression.
pKa = - log Ka
The larger the Ka, the smaller the pKa.
Examples:
Ka of acetic acid = 1.8 x 10^-5; pKa = - log (1.8 x 10^-5) = 4.74
Ka of nitrous acid = 3.2 x 10^-8; pKa = - log (3.2 x 10^-8) = 7.5
Acetic acid is the stronger acid with a larger Ka value but with smaller pKa value.

The strongest acid from your list is HNO2 (pKa = 7.5)
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