How to calculate pKa from pH?

  In calculating the (pKa ) acid ionization /dissociation constant of
a compound, you need the following steps.

Step 1

Define the acid and its conjugate base in the relevant reaction. The acid is the compound with the extra
hydrogen, and its conjugate base is the compound without that hydrogen. That is,
it’s the compound form when  a
corresponding acid loses an hydrogen atom.. For instance, Ethanoic acid has a formula
of CH3COOH; its conjugate base loses one ionisable hydrogen atom to its
corresponding base such as CH3COO-.

Step 2

 You should carefully write down all of the
information that the problem gives you. To successfully solve for the pKa, the
problem should include the pH of your solution, the concentration of the
relevant acid and the concentration of its conjugate base. You should already
have recognized the acid and its conjugate base in Step 1.

Step 3

Divide the concentration of acid by
the concentration of the conjugate base. For a solution that contains 1M ethanoic
acid  and 0.5M ethanol, you would divide
1 by 0.5 to get 2.

Step 4

Take the log of the answer that you
obtain in Step 3. Log is short for logarithm, which is a mathematical function.
Nonetheless, you will virtually at all times need a calculator to find logs. In
this example, you would use a calculator to take the log of 2 and get 0.30.

Step 5

Add the number that you got in Step
4 to the pH given to you in the problem. If your solution has a given pH of
1.3, add 0.3 to 1.3 to get a pKa of 1.6.

Step 6

Crisscross your work. The most crucial
formula to remember is that the pKa = pH + log(Acid/Base), better known as the
Henderson-Hasselbalch equation. As long as your numbers do not contradict the
Henderson-Hasselbalch equation, you have correctly calculated the pKa of your
solution.