Limitations to the Henderson-Hasselbach Eq

[sid bauer]

What were the limitations for the Henderson-Hasselbach equation? In other words, when can it NOT be calculated? (i.e. when acids/bases are strong/weak)

Also, to get rid of the logs, you can simply raise everything to the power of 10 correct? I've used it for other equations but I'm not sure if you can apply it to this equation...

Thank you!

[Ben Rolnik 1D]

This is a really good question....

The truth is that the Henderson-Hasselbach Eq is actually just a manipulation of the basic equilibria equation.... therefore, instead of thinking about it in terms of its limitations and usefulness, it's more important to think about the general equation itself and how to manipulate it to solve for terms you need.

pH = pKa + Log (A / HA)

is the same thing as:

Ka = [H}[A] / [HA]

when you solve for -log([H])....

Thus.... when you're given a problem and the kA is sufficiently small (below E - 4), you can essentially just use the Hasselbach Eq to solve....

Again the most important thing is thinking about it in terms of what is needed in an actual problem. If you can find more specific rules, I would love to know! So far, for me, I've had to just kind of take each problem on a case by case basis.