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How do you get a Pka?


How do you get a Pka? 

Step 1

Dissociation constant (Ka) is the equilibrium constant of a reaction which depicts the dissociation of a proton from a weak acid. 

For example, a weak acid i.e. HA dissociates as per following reaction:

HA  + H2O <-----> H3O+ + A-

The Ka of above reaction can be written as:



Image Transcriptionclose

[Н:0'JA] К. [НA]

Step 2

pKa  stands for the negative logarithm (to base 10) of Ka . 

Mathematically, it is written as pKa  = -log (Ka ).

So, to calculate pKa  , negative logarithm of Ka  is to be calculated.

Step 3

pKa can also be observed on titration curve of weak acid with strong base. 

On plotting the curve between the pH of solution and volume of strong base added, the pH at half equivalence point is the pKa.

Equivalence point is that at which the pH of the solution rises abruptly on adding small amount of base. At this point, the moles of weak acid are equal to the moles of strong base in the reaction mixture.

Half equivalence point is that at which &nb...

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