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A $10^{-8} \mathrm{M}$ solution of HCl has a pH of 8

   A $10^{-8} \mathrm{M}$ solution of HCl has a pH of 8

Molecular Biology of the Cell

Molecular Biology of the Cell

Bruce Alberts,… 5th Edition

Chapter 2, Problem 2

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Solved by Expert Joanna Quigley

03/10/2021

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We know that the concentration of HCl is $10^{-8} \mathrm{M}$. Show more…

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A $10^{-8} \mathrm{M}$ solution of HCl has a pH of 8

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Key Concepts

pH

pH is defined as the negative logarithm of the hydrogen ion concentration in a solution and is used to measure its acidity or basicity. It provides a convenient scale where lower values indicate acidic conditions and higher values indicate basic conditions, with pH 7 representing neutral water. In calculations, pH is affected by contributions from all sources of H? ions in the solution.

Strong Acid Behavior

A strong acid, like HCl, is characterized by its complete dissociation in water, meaning that nearly every molecule of the acid releases a hydrogen ion. This behavior ordinarily makes the pH of a strong acid solution directly proportional to its concentration. However, when the concentration is extremely low, other factors such as water’s own ionization become significant.

Autoionization of Water

Water undergoes autoionization, a process in which a very small number of water molecules dissociate into hydrogen ions (H?) and hydroxide ions (OH?). In pure water at 25°C, the concentration of H? (and OH?) is approximately 10?? M. In very dilute acid solutions, the hydrogen ions contributed by this autoionization can be comparable to, or even exceed, the ions from the acid itself, affecting the overall pH.

pH Calculation in Extremely Dilute Acidic Solutions

When dealing with extremely dilute solutions of strong acids, the contribution of hydrogen ions from water's autoionization must be taken into account along with the ions from the acid. This interaction can lead to counterintuitive results, such as a solution of a strong acid displaying a pH that does not seem as acidic as expected. Proper analysis involves adding the hydrogen ion concentrations from both the acid and water, and then converting the total concentration into the pH value.

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