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Strong Acid Strong Base

This is the most important rule to know for acid-base reactions:

Stronger acid plus stronger base gives weaker acid plus weaker base.

The relative strengths of the starting acid and the final acid (i.e. the “conjugate acid”) give you a rough idea of how favored it should be.

How do you tell if an acid is stronger or not? Well, to be honest it really helps to have a pKa table. Some instructors say, “don’t memorize pKas”, but I think it’s essential to have this list in your head – at least these 10 rough values. You wouldn’t want to play poker without knowing the value of the hands, right? You’ll find this useful all through Org 1 and Org 2.

HCl: –8 (stongest acid on this list)

H2SO4: -3

H3O(+): –2

Acetic acid: about 5

NH4(+):  10

Water, alcohols: about 16-18

Acetylene: 25

H2: about 35 35

NH3: 38

Alkanes: 50 (weakest acid on this list)


Here’s a quick example.

Tomorrow: how acidity and basicity are related.

Thanks for reading! James

P.S. For a more detailed walkthrough, see this post: “How to Use a pKa table