Answer
In a pure water solution, the $[H_3O^+]$ produced by the autoionization is equal to $1.0 \times 10^{-7}$, which is already a very small number.
When acid is added to water, it will produce more $H_3O^+$, and according to Le Chatelier's Principle, the autoionization of water will shift to the left, producing even less hydronium ion (since more product was added) from this reaction:
$$2H_2O(l) \leftrightharpoons H_3O^+(aq) + OH^-(aq)$$
Therefore, the contribution of the autoionization of water is very small in most acid solutions and can be neglected.
Work Step by Step
In a pure water solution, the $[H_3O^+]$ produced by the autoionization is equal to $1.0 \times 10^{-7}$, which is already a very small number.
When acid is added to water, it will produce more $H_3O^+$, and according to Le Chatelier's Principle, the autoionization of water will shift to the left, producing even less hydronium ion (since more product was added) from this reaction:
$$2H_2O(l) \leftrightharpoons H_3O^+(aq) + OH^-(aq)$$
Therefore, the contribution of the autoionization of water is very small in most acid solutions and can be neglected.
