What is pKa?
pKa is the criterion used to determine the acidity of the molecule. It is used to measure the acid strength. The lesser the pKa is, the molecule would hold proton less tightly; hence the more potent the acid will be.
Ka is the acid dissociation constant. It is used to determine how much an acid dissociates in solution. The larger the Ka, the higher would be dissociation. Hence stronger would be the acid.
pKa is related to Ka as,
pKa = – log[Ka]
Table of Content
- How to calculate pKa
- pKa from Ka or Concentration
- pKa from pH
- pKa from the Titration Curve
- Frequently Asked Questions – FAQs
How to calculate pKa
pKa is the criterion used to determine the acidity of the molecule. It is used to measure the acid strength. The lesser the pKa is, the molecule would hold proton less tightly; hence the more potent the acid will be.
Numerous parameters can determine the pKa value. Few of them are enlisted below.
- pKa from Ka or Molarity
- pKa from pH
- pKa from the titration curve
pKa from Ka or Concentration
Consider dissociation of acid HX,
HX ⥦ H+ + X–
For the above equation, Ka would be
We know that Ka and pKa are related.
pKa= – logKa
Here, the quantities in the brackets symbolise the concentration.
Thus we can quickly determine the pKa value if the concentration of reactants and products or Ka are known.
pKa from pH
From the Henderson equation of acidic buffer, we can quickly determine the value of pKa from the pH.
pH = pKa + log {[salt] / [Acid]}
Let [salt] / [Acid] be equal to 10 then,
pH = pKa + log 10
pH = pKa + 1
Let [salt] / [Acid] be equal to 1 / 10 then,
pH = pKa + log 1 / 10
pH = pKa + log 1 – log 10
pH = pKa – 1
Thus we can quickly determine the pKa value if the pH of the solution is known.
pKa from the Titration Curve
pKa value can be determined by the titration curve.
To calculate the pKa of the solution, firstly, we will determine the equivalence point and then find the pH of the solution. pKa of the solution is equivalent to the pH of the solution at its equivalence point.
Hence we can quickly determine the value of pKa by using a titration curve.
Frequently Asked Questions on How to calculate pKa
What is pKa?
pKa is the criterion used to determine the acidity of the molecule. It is used to measure the acid strength. The lesser the pKa is, the molecule would hold proton less tightly; hence the more potent the acid will be.
pKa is related to Ka as,
pKa = -log[Ka]
What is Ka?
Ka is the acid dissociation constant. It is used to determine how much an acid dissociates in solution. The larger the Ka, the more would be dissociation. Hence stronger would be the acid.
pKa is related to Ka as,
pKa = -log[Ka]
How can we calculate the pKa value from pH?
From the Henderson equation of acidic buffer, we can quickly determine the value of pKa from the pH.
pH = pKa + log {[salt] / [Acid]}
Let [salt] / [Acid] be equal to 10 then,
pH = pKa + log 10
pH = pKa + 1
Let [salt] / [Acid] be equal to 1 / 10 then,
pH = pKa + log 1 / 10
pH = pKa + log 1 – log 10
pH = pKa – 1
Thus we can quickly determine the pKa value if the pH of the solution is known.
How can we calculate the pKa value from the titration curve?
To calculate the pKa of the solution, firstly, we will determine the equivalence point and then find the pH of the solution. pKa of the solution is equivalent to the pH of the solution at its equivalence point.
Hence we can quickly determine the value of pKa by using a titration curve.
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