Final answer:
The sentence should be completed as: 'The lower the pH, the higher the concentration of H₃O⁺ ions.' A lower pH indicates a higher concentration of hydronium ions, making the solution more acidic.
Step-by-step explanation:
The lower the pH, the higher the concentration of H₃O⁺ ions. pH and concentration of hydronium ions (H₃O⁺) are inversely related in aqueous solutions. As per the pH scale, pure water has a pH of 7 and is considered neutral. Acids have a pH lower than 7, which means they have a higher concentration of hydronium ions than pure water, hence they are more acidic. Conversely, bases have a pH higher than 7 and a lower concentration of hydronium ions. The pH value decreases as the concentration of H₃O⁺ ions increases, due to the mathematical relationship in the pH calculation that involves the negative logarithm of the hydronium ion concentration.