The pH of the substance is calculated through the equation,
pH = -log[H⁺]
This means that the pH of a certain substance is the negative log of the concentration of the H⁺ ions. In this item, we are given with the pH of the substance.
Substituting the known value of pH,
9.15 = -log[H⁺]
The value of [H⁺] is 7.08 x 10⁻¹⁰.