Question

Can the pH of a solution of \(\mathrm{HCl}\) ever be greater than 7 at \(25^{\circ} \mathrm{C}\) ? Explain.

Short answer

No, the pH of a solution of \(\mathrm{HCl}\) cannot be greater than 7 at \(25^{\circ} \mathrm{C}\). This is because \(\mathrm{HCl}\) is a strong acid that dissociates completely in water, contributing \(\mathrm{H+}\) ions which lower the pH, making the solution acidic.

Step-by-step solution

Understanding Hydrochloric Acid in Water

When \(\mathrm{HCl}\) is dissolved in water, it dissociates completely into hydrogen ions (\(\mathrm{H^{+}}\)) and chloride ions (\(\mathrm{Cl^{-}}\)). The hydrogen ions increase the acidity of the solution, which is reflected in a decrease in pH.

Considering the pH Scale

The pH of a solution is calculated as \(-\log[\mathrm{H^{+}}]\). An increase in \(\mathrm{H^{+}}\) ions from the dissociation of \(\mathrm{HCl}\) results in a decrease in pH, making the solution more acidic. So the pH would be less than 7, the neutral point on the scale.

Conclusion

Given that \(\mathrm{HCl}\) is a strong, fully dissociating acid, its presence in water only adds \(\mathrm{H^{+}}\) ions, reducing the pH and making the solution more acidic, not basic. Therefore, it is not possible for a solution of \(\mathrm{HCl}\) to have a pH value greater than 7.

Key concepts

Hydrochloric Acid Properties

Hydrochloric acid, often abbreviated as HCl, is a colorless, pungent solution of hydrogen chloride in water. It's a strong, monoprotic acid, which means it can release one hydrogen ion \( \( \mathrm{H^{+}} \) \) per molecule in aqueous solution. This ability underlies its high reactivity and effectiveness in applications ranging from laboratory analysis to industrial manufacturing and food processing.

When \( \( \mathrm{HCl} \) \) is dissolved in water, it ionizes completely, producing hydrogen ions ( \( \( \mathrm{H^{+}} \) \) ) and chloride ions ( \( \( \mathrm{Cl^{-}} \) \) ). In its gaseous form, hydrogen chloride is a potent acid gas that is hazardous and corrosive. However, hydrochloric acid, when properly diluted, plays a vital role in digestion as stomach acid, helping to break down and digest food efficiently. Safety in handling is critical since the acid can be destructive to materials and harmful to skin, eyes, and mucous membranes.

pH Scale Interpretation

The pH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It ranges typically from 0 to 14, with 7 being the neutral point, which is the pH of pure water at \(25^\circ \)C. Solutions with a pH less than 7 are considered acidic, while those with a pH greater than 7 are basic (alkaline).

The pH value is calculated using the formula \( -\log[\mathrm{H^{+}}] \) where \( [\mathrm{H^{+}}] \) is the concentration of hydrogen ions in moles per liter. Due to the logarithmic nature of the scale, each whole pH value below 7 is ten times more acidic than the next higher value. For instance, a solution with a pH of 3 is ten times more acidic than one with a pH of 4. This logarithmic relationship means that small changes in pH represent significant changes in acidity.

Strong Acid Dissociation

In the context of acid-base chemistry, a strong acid is characterized by its complete dissociation in water. This means when a strong acid, like hydrochloric acid, is added to water, it rapidly donates its hydrogen ions to the water, producing \( \(\mathrm{H^{+}} \) \) ions and leaving behind the conjugate base, in the case of HCl, chloride ions \( \(\mathrm{Cl^{-}} \) \).

The complete dissociation of strong acids is a one-way process under standard conditions, and this attribute equates to a high level of conductivity due to the increased density of ions, which is not the case for weak acids that only partially dissociate in solution. Strong acids have low pH values (less than 7), further demonstrating their high acidity. Examples of strong acids, besides hydrochloric acid, include sulfuric acid ( \( \(\mathrm{H_{2}SO_{4}} \) \) ) and nitric acid ( \( \(\mathrm{HNO_{3}} \) \) ). The strong dissociation property is why these acids are frequently used in reactions requiring a significant and consistent acid concentration.