Why do we use expression of Ka for weak acids more often than strong acids

Question

Why do we use expression of Ka for weak acids more often than strong acids

Answer 1

Equilibrium constants (Ka's) are a ratio of the concentration of ionized material to unionized material. Strong acids ionize completely, so they all have the same Ka. Not all weak acids ionize to the same extent, so they have Ka's which differ.

Answer 2

We use Ka to describe the extent of dissociation of protons (which form hydronium ions in solution) from a particular acid--Related to the strength of the acid!  

If you recall, strong acid are often said to, "completely dissociate" in water.  Others say, "completely ionize", whichever is fine.  If an acid completely ionizes/dissociates, the Ka is:

HA -->H+ + A-

Ka = Big Huge Number / 0 : by definition is infinity!

This is not quite the truth, we all know that there will be a small amount of HA still not dissociated.  What matters is that the amount is so small, it is not significant--In other words percentages less than measurable and below the uncertainty of any physical measurement!  Now, if you look up Ka values for strong acids, say HCl, you will get no where fast--People often report pKa instead.  One will find values of pKa for strong acids between -1 to -7, even for the same strong acid.  

Lets do the math:

if pKa = -1 to -7 (even -1 sometimes), then:  10^1 to 10^7 or   Ka = 10 to 10000000

Or

[H+][A-]/[HA]=10 to 10000000

so there is at least 10 to 10000000 as much H+ as there is HA--Usually 10 times is so much different we are talking a significant amount more (often called "an order of magnitude" in fancy chemistry talk).

So Ka values aren't often used for strong acids because they are uncertain.  Some are not even based on physical measurement, in fact most aren't.  Instead, they are based on theoretical calculations and models--Which may or may not be accurate (all based on assumptions etc.).  

In contrast, for weak acids, the values are, very often, physically measurable.  Also, the amount of H+ can be measured with greater certainty, so the physical measurement is quite accurate.  

Ka values of weak acids just matter more.  They have more of a practical/physical significance--So use em'!