## Why is H-H Equation an approximation

Crystal Ma
Posts: 9
Joined: Fri Sep 25, 2015 3:00 am

### Why is H-H Equation an approximation

I think I heard prof say Henderson-Hasselbalch Equation is a way to get the approximate pH. I didn't get why. Would anybody explain?

Also why the beginning of the pH curve of a strong acid-strong base titration flatter than a strong acid-weak base titration?

Thank you.

804584179
Posts: 32
Joined: Fri Sep 25, 2015 3:00 am

### Re: Why is H-H Equation an approximation

strong acid weak base has a buffer zone (buffer resist ph change), as opposed to strong acid strong base titration

Sarah Noorani 3L
Posts: 30
Joined: Fri Sep 25, 2015 3:00 am

### Re: Why is H-H Equation an approximation

When we use the Henderson-Hasselach equation, we assume that the concentration of the acid, [HA] is the initial acid concentration. This is an approximation because a small percentage of the acid is converted to its conjugate base in the reaction HA + H2O --> A- + H3O+. Since we do not use the concentration of the acid after this reaction (since its initial concentration changes by a very small, almost insignificant amount), the acid concentration is an approximation and, therefore, the Henderson-Hasselbach gives us only an approximation of the pH of the solution.