Which of these correctly defines the pH of a solution?

A. the negative log of the hydronium ion concentration

B. the log of the hydronium ion concentration

C. the negative log of the hydroxide ion concentration

C. the log of the hydroxide ion concentration

First, we recall what a hydronium ion is. The hydronium ion is H₃O⁺. The hydronium ion is formed when a hydrogen ion (H+) is added to a water molecule. Therefore, the concentration of hydronium ion is equivalent to the concentration of the hydrogen ion concentration. Conventionally, the definition of pH is that it is the negative log of the hydrogen ion concentration. As we have seen that the hydronium ion concentration is equivalent to the hydrogen ion concentration, the answer is A.