Acids and Bases: Calculating pH of a Strong Acid

A strong acid is one that completely dissociates into its ions in water. This makes calculating the hydrogen ion concentration, which is the basis of pH, easier than for weak acids. Here's an example of how to determine the pH of a strong acid.

pH Question

What is the pH of a 0.025 M solution of hydrobromic acid (HBr)?

Solution to the Problem

Hydrobromic Acid or HBr is a strong acid and will dissociate completely in water to H⁺ and Br^-. For every mole of HBr, there will be 1 mole of H⁺, so the concentration of H⁺ will be the same as the concentration of HBr. Therefore, [H⁺] = 0.025 M.

pH is calculated by the formula

pH = - log [H⁺]

To solve the problem, enter the concentration of the hydrogen ion.

pH = - log (0.025)
pH = -(-1.602)
pH = 1.602

Answer

The pH of a 0.025 M solution of Hydrobromic Acid is 1.602.

One quick check to make sure your answer is reasonable is to verify the pH is closer to 1 than to 7 (certainly not higher than this.) Acids have a low pH value. Strong acids typically range in pH from 1 to 3.