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### Why is H-H Equation an approximation

Posted: Fri Dec 04, 2015 7:22 pm
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.

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

Posted: Fri Dec 04, 2015 7:51 pm
strong acid weak base has a buffer zone (buffer resist ph change), as opposed to strong acid strong base titration

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

Posted: Sat Dec 05, 2015 1:19 am
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.