Question

A solution of an acid in water has a pH of 4, which is slightly acidic. Is this solution a weak acid?

Answer 1

Possibly, but we don't really know.

Explanation:

For the protonolysis of an acid in water we write:

`HA(aq) + H_2O(l) rightleftharpoons H_3O^+ + A^-`

Now for a strong acid, the equilibrium lies strongly to the right; whereas for a weak acid, the equilbrium lies to the left. However, what if I take a dilute solution of a strong acid?

Suppose I have prepared `0.0001` `HNO_3(aq)`; in water, `[H_3O^+]=[NO_3^-]=10^-4*mol*L^-1`, but `pH=4` by definition. (`pH=-log_10[H_3O^+]`)