What is the equation used to calculate pH?

The equation used to calculate pH is based on the concentration of hydronium ions ((H_3O^+)) in a solution. The formula to determine pH is given by

[ \text{pH} = -\log[H_3O^+] ]

This formula indicates that pH is the negative logarithm of the molar concentration of hydronium ions. The reason this is the correct option is due to the relationship between acidity and hydronium ion concentration in aqueous solutions. A higher concentration of (H_3O^+) correlates with a lower pH value, indicating a more acidic solution.

The other choices relate to concepts surrounding pH but do not define how to directly calculate pH. For instance, the relationship between pH and pOH can be expressed through

[ \text{pH} + \text{pOH} = 14 ]

This equation provides a way to find the pH if you know the pOH but does not directly give the pH value itself. The choice concerning pOH being equal to the negative logarithm of (H_3O^+) is not accurate as it mistakenly interchanges the