An aqueous solution has a hydronium ion concentration of #6 * 10^-9# mole per liter at 25°C. What is its pH?
The pH of a solution could be calculated using the concentration of hydronium
Acids & Bases | Nature, Strength & pH Scale.
So what is
Here it is simply the function
It is worthwhile to review the logarithmic function, and see why the use of logarithms (and slide rules) was absolutely widespread among scientists and engineers in a pre-calculator world.