Understanding Solution pH and Acidity

The pH scale is used to specify the acidity or basicity of an aqueous solution. It is defined as the negative base-10 logarithm of the activity of the hydrogen ion (\(\text{H}^+\)). In dilute solutions, the activity of the hydrogen ion is approximately equal to its concentration, expressed in molarity (\(\text{mol/L}\)). The formula for pH is:

\[ \text{pH} = -\log_{10}[\text{H}^+] \]

A solution's pH tells us whether it is acidic, neutral, or basic:

• Acidic Solution: pH < 7. This means the concentration of \(\text{H}^+\) ions is higher than \(\text{OH}^-\) ions.
• Neutral Solution: pH = 7. The concentration of \(\text{H}^+\) ions is equal to the concentration of \(\text{OH}^-\) ions (at 25°C). Pure water is neutral.
• Basic (Alkaline) Solution: pH > 7. This means the concentration of \(\text{OH}^-\) ions is higher than \(\text{H}^+\) ions.

Strong acids and strong bases completely dissociate in water, leading to significant changes in pH. Salts, formed from the reaction of acids and bases, can produce neutral, acidic, or basic solutions depending on the strength of the parent acid and base. This phenomenon is called salt hydrolysis.

Salt Hydrolysis and Solution pH

Salt hydrolysis is the reaction of a salt with water to produce an acidic or basic solution. The pH of a salt solution depends on the nature of the parent acid and base from which the salt is formed.

• Salt of Strong Acid and Strong Base: These salts do not undergo significant hydrolysis because the ions are weak conjugate base/acid respectively and do not react appreciably with water. The solution is neutral (pH = 7). Example: \(\text{NaCl}\) (from \(\text{HCl}\) and \(\text{NaOH}\)), \(\text{KCl}\) (from \(\text{HCl}\) and \(\text{KOH}\)).
• Salt of Strong Acid and Weak Base: The cation from the weak base hydrolyzes water, releasing \(\text{H}^+\) ions. The solution is acidic (pH < 7). Example: \(\text{NH}_4\text{Cl}\) (from \(\text{HCl}\) and \(\text{NH}_4\text{OH}\)), \(\text{FeCl}_3\) (from \(\text{HCl}\) and \(\text{Fe(OH)}_3\)).
• Salt of Weak Acid and Strong Base: The anion from the weak acid hydrolyzes water, releasing \(\text{OH}^-\) ions. The solution is basic (pH > 7). Example: \(\text{CH}_3\text{COONa}\) (from \(\text{CH}_3\text{COOH}\) and \(\text{NaOH}\)).
• Salt of Weak Acid and Weak Base: The pH depends on the relative strengths (dissociation constants, \(\text{K}_a\) and \(\text{K}_b\)) of the parent acid and base. If \(\text{K}_a > \text{K}_b\), the solution is acidic; if \(\text{K}_b > \text{K}_a\), it's basic; if \(\text{K}_a \approx \text{K}_b\), it's nearly neutral.

Analyzing the Given Solutions

Let's examine each option to determine its nature and predicted pH:

Identifying the Solution with pH less than 7

Based on the analysis, we are looking for a solution with a pH less than 7, which indicates an acidic solution. \(\text{NaOH}\) is a strong base, so its pH will be greater than 7. \(\text{KCl}\) is a salt of a strong acid and a strong base, resulting in a neutral solution with a pH of 7. \(\text{NaCl}\) is also a salt of a strong acid and a strong base, resulting in a neutral solution with a pH of 7. \(\text{FeCl}_3\) is a salt of a strong acid (\(\text{HCl}\)) and a weak base (\(\text{Fe(OH)}_3\)). The hydrolysis of the \(\text{Fe}^{3+}\) ion produces \(\text{H}^+\) ions, making the solution acidic.

Therefore, the solution of \(\text{FeCl}_3\) will have a pH less than 7.

Revision Table: pH and Solution Types

Additional Information on Salt Solutions pH

The pH of a salt solution is determined by the extent to which its constituent ions react with water (hydrolyze). Ions from strong acids and bases are very weak conjugate bases and acids, respectively, and their hydrolysis is negligible. For instance, \(\text{Na}^+\) (from strong base \(\text{NaOH}\)) and \(\text{Cl}^-\) (from strong acid \(\text{HCl}\)) do not react significantly with water.

However, ions derived from weak acids or bases are stronger conjugate bases or acids, and they do hydrolyze:

• A cation from a weak base (like \(\text{Fe}^{3+}\) from \(\text{Fe(OH)}_3\)) acts as a weak acid and reacts with water: \[ \text{Fe}^{3+}\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{Fe(OH)}^{2+}\text{(aq)} + \text{H}^+\text{(aq)} \] This process releases \(\text{H}^+\) ions, lowering the pH. Iron(III) ions can undergo further hydrolysis steps, but the overall effect is the production of acidic solution.
• An anion from a weak acid (like \(\text{CH}_3\text{COO}^-\) from \(\text{CH}_3\text{COOH}\)) acts as a weak base and reacts with water: \[ \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{CH}_3\text{COOH(aq)} + \text{OH}^-\text{(aq)} \] This process releases \(\text{OH}^-\) ions, raising the pH.

Understanding the parent acid and base is crucial for predicting the pH of a salt solution.