how is the concentration of hydronium ions affected when a solution of an acid is diluted

The concentration of hydronium ions (H3O+) decreases when a solution of an acid is diluted. This happens because dilution increases the total volume of the solution while the amount of acid (and thus the amount of hydronium ions) remains the same. Since concentration is the amount of solute per unit volume, spreading the same number of hydronium ions over a larger volume reduces their concentration. Additionally, dilution may shift the acid dissociation equilibrium slightly, but the net effect is a decrease in hydronium ion concentration due to the increase in volume and resulting lower molarity of the acid solution.