Strong and Weak Acids and Bases

The vast majority of all acids and bases are weak in strength. This means that very few actively donate or accept protons (H⁺ ions). Strong acids dissociate completely in aqueous solutions. HCl, for example, is a very strong acid and it separates completely into H⁺ and Cl^-. There is essentially no HCl left in solution. The same rule goes for strong bases... you are left with a lot of hydroxide ions and the conjugate acid floating about, but practically none of the original substance remains.

Weak acids do not dissociate very much. For instance, acetic acid, CH₃COOH, is a weak acid. The K_a is rather small, 1.8 x 10^-5. This means that when the concentrations of the products, H⁺ and CH₃COO^-, are multiplied together and divided by the original concentration of acetic acid, you get 1.8 x 10^-5. What this says is that very little of the acetic acids separates into H⁺ and CH₃COO^-. The same is true for weak bases. Their K_b values are very small because so little of the reactant dissociates into the products.

Here's a quick way to tell the relative strength or weakness of an acid or base using K_a and K_b:

Acid Strength	Ka	Conjugate Base       Strength	Kb
Strong	> 1	very weak	< 10-16
Weak	1 to 10-16	weak	1 to 10-16
Very weak	< 10-16	strong	> 1

A little secret...
...This is why that perchloric acid question in our interviews with the different chemists was a trick question... The dissociation constant for perchloric acid is so large, no one's bothered to measure it!

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