Question

What is the approximate percentage of a weak acid (pKa = 5.4) in the non-ionized form in plasma having a pH of 7.4?

Answer 1

For any acid HA, we have the dissociation reaction as,

By the definition of dissociation constant K_a, we have,

Taking negative log, we will have,

Now it is given that pH=7.4 and pK_a = 5.4.

Putting all these, we have,

Let initial concentration of acid be

Let the fraction dissociated be

Then at equilibrium,

Using these, we have,

Thus, Acid in non-ionized form i.e. HA will be,

% of non-ionized acid = 0.0099 c / c *100 = 0.99 % (Answer)