### pKa and pKb

Posted:

**Wed Jan 22, 2020 10:21 am**why does a smaller pKa or pKb correspond to a stronger acid and strong base?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=49&t=57004

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Posted: **Wed Jan 22, 2020 10:21 am**

why does a smaller pKa or pKb correspond to a stronger acid and strong base?

Posted: **Wed Jan 22, 2020 12:32 pm**

A strong acid would have a higher Ka. For example, a strong acid could have Ka = 10^-1. The formula for pKa is pKa = -logKa. If you do -log(10^-1), you get pKa = 1. Therefore this strong acid has a low pKa.

I like to first think about Ka and then work backwards. A higher Ka means 10^-x where x is a small integer like 1, 2, etc. We know that -log(10^-x) is equal to x, which means a stronger acid has a lower pKa.

I hope this helps.

I like to first think about Ka and then work backwards. A higher Ka means 10^-x where x is a small integer like 1, 2, etc. We know that -log(10^-x) is equal to x, which means a stronger acid has a lower pKa.

I hope this helps.

Posted: **Wed Jan 22, 2020 12:44 pm**

the function p has a negative sign, resulting in larger inputs having smaller outputs. Thus, a larger Ka (and stronger acid) would have a smaller pKa.