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Equivalence point for titrations

April 8, 2019

 

Titration is a lab technique where a known acid/base is added to unknown acid or base to find out its molarity. When stoichiometrically moles of acid are equal to moles of base, the equivalence point has been reached. 

A common question about the equivalence point is to find the pH of solution at that point. Let's break our titration into three categories.

 

Strong acid with strong base titration:

When a strong acid reacts with a strong base, water and neutral salt is created. The equivalence point of the strong acid/strong base titration has a pH of 7.

 

Strong acid with weak base titration:

When a strong acid reacts with weak base, water and acidic salt is produced. At equivalence point, all of strong acid and weak base is gone. However, since through the reaction, weak conjugate acid is produced, pH at the equivalence point is slightly acidic. To calculate pH, we need to do the weak acid calculation using the ICE chart.

 

Strong base with weak acid titration: 

When a strong base reacts with weak acid, water and basic salt is produced. At equivalence point, all of weak acid and strong base is gone. However, since through the reaction, weak conjugate base is produced, pH at the equivalence point is slightly basic. To calculate pH, we need to do the weak base calculation using the ICE chart.

 

Therefore, when looking at titration curve, we can see, based on the pH at the equivalence point what kind of titration took place. 

 

For more help with general chemistry, AP chemistry, please contact Transformation Tutoring and speak to one of our amazing chemistry tutors in NYC, Brooklyn or online by calling 646-407-9078.

 

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