To calculate the pH of an aqueous solution, what must be known?

To calculate the pH of an aqueous solution, knowing the concentration of the hydronium ion is essential. pH is defined as the negative logarithm (base 10) of the hydronium ion concentration in the solution. The formula used is:

[ \text{pH} = -\log[\text{H}_3\text{O}^+] ]

where ([\text{H}_3\text{O}^+]) represents the molar concentration of hydronium ions. This relationship indicates that an increase in hydronium ion concentration results in a lower pH value, which signifies a more acidic solution. Conversely, a decrease in hydronium ion concentration leads to a higher pH, indicating a more basic solution.

While factors such as temperature can influence the pH of a solution by affecting ion dissociation, it is specifically the hydronium ion concentration that directly determines the pH value itself. Therefore, knowing the hydronium ion concentration is paramount for calculating the pH accurately.