Nitric acid HNO3 is a strong acid that is completely ionized

Nitric acid (HNO3) is a strong acid that is completely ionized in aqueous solutions of concentrations ranging from 1% to 10% (1.50 M ). However, in more concentrated solutions, part of the nitric acid is present as un-ionized molecules of HNO3. For example, in a 50% solution (7.50 M ) at 25°C, only 33% of the molecules of HNO3 dissociate into H+ and NO3–. What is the value of Ka for HNO3?

Solution

HNO3(aq) <==> H+ + NO3^-

When change in the concentration ie x = 7.5 * 0.33 = 2.475 M

occurs then using ICE TABLE, we have:

HNO3(aq)...... <==>....... H+......... + NO3^-

7.50M ...... ........ ....... 0...................... . 0 ........... initial
- 2.475.......... ........ +2. 475....... +2.475...... Change

5.025 ....... ........... +2. 475..... ...+2.475...... Equilibrium

Ka = [H+][NO3-] / [HNO3]
Ka = (2.475)² / 5.025
Ka = 1.21

Since ka > 1, so products will be favored in this equilibrium.