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# How do I calculate the pH for a strong acid solution after dilution?

How does the pH of a typical 3 ounces of gastric hydrochloric acid (HCl) with a pH of 1.0 change when 16 ounces of distilled water is added?
Bill 12/20/99

 Vocabulary dilution molarity pH
1. Estimate the initial hydrogen ion concentration. A simple, working definition of pH is pH = - log[H+], where [H+] is the hydrogen ion molarity. To obtain the hydrogen ion molarity from the pH, remember that a logarithm of a number is really just the exponent when that number is written as a power of ten. The definition of pH solved for hydrogen ion molarity is then [H+] = 10-pH. For example, the molarity of hydrogen ions in a pH 5 solution is 10-5 M.
2. Compute the hydrogen ion concentration after dilution. If the solution is diluted to twice its original volume, the concentration will be cut in half. If the solution is diluted to three times its original volume, the concentration will be cut to one third; and so on. If the original volume is V1 and the total volume after dilution is V2, the final concentration will be V1/V2 times the original concentration.
3. Convert hydrogen ion concentration back to pH, using the definition for pH.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How do I calculate the pH for a strong acid solution after dilution?