Acids and Bases - Calculating pH of a Strong Acid

pH of Strong Acid Worked Chemistry Problems

You can't just dip litmus paper into a strong acid to test its pH.
You can't just dip litmus paper into a strong acid to test its pH.

A strong acid is one which 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 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 provlem, 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 you can do, 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.

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Helmenstine, Todd. "Acids and Bases - Calculating pH of a Strong Acid." ThoughtCo, May. 29, 2016, thoughtco.com/calculating-ph-of-a-strong-acid-problem-609587. Helmenstine, Todd. (2016, May 29). Acids and Bases - Calculating pH of a Strong Acid. Retrieved from https://www.thoughtco.com/calculating-ph-of-a-strong-acid-problem-609587 Helmenstine, Todd. "Acids and Bases - Calculating pH of a Strong Acid." ThoughtCo. https://www.thoughtco.com/calculating-ph-of-a-strong-acid-problem-609587 (accessed December 11, 2017).