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 Ka 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 Kb 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 Ka and Kb:
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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!