**pH of solutions of very weak acids**

If the acid is very weak then the extent of dissociation will be very small and for all practical purposes [HA] in the equation may be taken as the initial concentration of the acid. As [H^{+}] = [A^{–}] equation reduces to

[H^{+}]^{2} = K_{a} [HA]

so that, **[H ^{+}] = √{K_{a} [HA]}** … … … (1)

This relation may be used to make approximate calculations of pH of solutions of very weak and moderately weak acids. If the pH of the solution of a weak acid is known an approximate value of its K_{a} may be calculated. The value of K_{a} may also be determined by a titration curve.

Example: Calculate the pH of a solution of n-butyric acid concentration 0.12 mol L^{-1} (K_{a} of the acid = 1.515 x 10^{-5} mol L^{-1}).

**Solution:** Using the relation (1), [H^{+}] = √{K_{a} [HA]}

= √[0.12×1.515×10^{-5}]

= 1.66 x 10^{-3}

pH = – log (1.66 x 10^{-3}) = 2.78.

pH is a measure of the concentration of hydrogen ions in a solution. Strong acids like hydrochloric acid at the sort of concentrations you normally use in the lab have a pH around 0 to 1. The lower the pH, the higher the concentration of hydrogen ions in the solution./sub