Question

What is meant by pH? True or false: A strong acid solution always has a lower \(\mathrm{pH}\) than a weak acid solution. Explain.

Short answer

pH is a measure of the acidity or basicity of an aqueous solution and is defined as the negative base 10 logarithm of the hydrogen ion concentration. Strong acids completely dissociate into their ions, resulting in a higher concentration of hydrogen ions and typically a lower pH value. Weak acids only partially dissociate, so they generally have higher pH values. However, the statement "A strong acid solution always has a lower pH than a weak acid solution" is not always true, as the pH value depends on both the concentration and the strength of an acid. In some cases, a concentrated weak acid may have a lower pH value than a very dilute strong acid.

Step-by-step solution

Definition of pH

pH is a scale that measures the acidity or basicity of an aqueous solution. It is defined as the negative base 10 logarithm of the concentration of hydrogen ions (\(H^{+}\)) or hydronium ions (\(H_3O^{+}\)) present in the solution. Mathematically, it can be represented as: pH = \(-log_{10} [H^{+}]\) or pH = \(-log_{10} [H_{3}O^{+}]\) A lower pH value indicates a higher concentration of hydrogen ions, which means the solution is more acidic. A higher pH value corresponds to a lower concentration of hydrogen ions, meaning the solution is more alkaline or basic. The pH scale ranges from 0 to 14, with 7 being neutral.

Strong Acids vs. Weak Acids

An important distinction between acids lies in their strength. A strong acid is an acid that completely dissociates into its ions in an aqueous solution, which means it donates all of its hydrogen ions or protons (\(H^{+}\)). As a result, a strong acid increases the concentration of hydrogen ions in the solution significantly. Examples of strong acids include hydrochloric acid (\(HCl\)), sulfuric acid (\(H_2SO_4\)), and nitric acid (\(HNO_3\)). A weak acid, on the other hand, only partially dissociates in an aqueous solution, resulting in a relatively lower concentration of hydrogen ions. Examples of weak acids include acetic acid (\(CH_{3}COOH\)), carbonic acid (\(H_{2}CO_{3}\)), and phosphoric acid (\(H_{3}PO_{4}\)).

Evaluating the Statement

The statement to evaluate is: "A strong acid solution always has a lower pH than a weak acid solution." While it is true that strong acids typically have lower pH values because they dissociate completely and increase the concentration of hydrogen ions in the solution, some weak acids may also have low pH values. The pH value depends on both the concentration and the strength of an acid. Different concentrations of strong and weak acids can result in situations where a relatively concentrated weak acid could have a lower pH value than a very dilute strong acid. Therefore, the statement is not always true, and we conclude that it is #false#.

Explanation

A strong acid solution does not always have a lower pH than a weak acid solution. Although strong acids do generally have lower pH values due to their complete dissociation, the pH value is also determined by the concentration of the acid. In some cases, a relatively concentrated weak acid could have a lower pH value than a very dilute strong acid. This highlights the importance of considering both concentration and strength when comparing the acidity of different solutions.