How is the pH of a solution determined?

The pH of a solution is determined by measuring the concentration of hydrogen ions (H⁺) present in that solution. This relationship is essential because pH is defined as the negative logarithm (base 10) of the activity of hydrogen ions. In mathematical terms, it can be expressed as pH = -log[H⁺]. As the concentration of hydrogen ions increases, the pH value decreases, indicating a more acidic solution. Conversely, a lower concentration of hydrogen ions corresponds to a higher pH, indicating a more basic (or alkaline) solution. Therefore, the critical factor in determining pH is the actual amount of hydrogen ions available in the solution, making this choice the correct one.

The other choices are related to different physical properties or characteristics of solutions but do not fundamentally influence or directly determine the pH level. For instance, the density of the solution involves mass and volume, while the temperature affects various chemical equilibria but is not a direct measure of hydrogen ion concentration. Similarly, knowing the mass of the solute can provide information about the concentration of the solution, but it does not directly translate to pH without additional context regarding the nature of the solute and its dissociation in solution.