Why Didn't They Use the H-H Equation in the End?

[Hidemons]

Here's a solved chemistry problem:

http://answers.yahoo.com/question/index?qid=20100731143735AAjKe10

Why did the answerer not use the H-H equation at the very end?
Please Help.


The H-H equation and the Ka= (H)(A)/(HA) equation (what was used in the problem) both use the same inputs and both are used to find pH. How do I pick one over the other?



(the template provided)

 

[epenguin]

Here's a solved chemistry problem:

http://answers.yahoo.com/question/index?qid=20100731143735AAjKe10

Why did the answerer not use the H-H equation at the very end?
Please Help.The H-H equation and the Ka= (H)(A)/(HA) equation (what was used in the problem) both use the same inputs and both are used to find pH. How do I pick one over the other?
(the template provided)

First he found [H⁺] then he took logs and found pH.

Ka= (H)(A)/(HA) is equivalent practically to the HH equation.

How to pick one over the other? Use K_a= (H)(A)/(HA) because, at the stage you're at you are going to understand clearly what you're doing better than if you just apply a (forgettable) formula; this will be needed for more difficult problems you might get next.

 

[Hidemons]

Ka= (H)(A)/(HA) is equivalent practically to the HH equation.

Practically or completely? I worked out the linked problem using H-H and it gives you a different answer. very frustrating

 

[epenguin]

Practically - you have to switch things about maybe.

the HH eq. is usually given in a form like

well see here: http://en.wikipedia.org/wiki/Henderson–Hasselbalch_equation

I don't know what you've done but someone like you easily gets pH and pK on the wrong side and mixes + and - log and (equivalently) gets the fraction upside down etc. That's why I recommend not using it, but sticking with something you understand easier.

 

[Borek]

Practically or completely? I worked out the linked problem using H-H and it gives you a different answer. very frustrating

Show what you did, answers should be identical. If they are different you are probably doing something wrong.

Henderson-Hasselbalch equation is just another form of Ka (see derivation on the page).