Why is H-H Equation an approximation

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

Why is H-H Equation an approximation

Postby Crystal Ma » 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.

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

Re: Why is H-H Equation an approximation

Postby 804584179 » 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

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

Re: Why is H-H Equation an approximation

Postby Sarah Noorani 3L » 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.


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