Question

Explain with equations and calculations, when necessary, whether an aqueous solution of each of these salts is acidic, basic, or neutral: (a) \(\left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4} ;\) (b) \(\mathrm{Na}_{2} \mathrm{SO}_{4} ;\) (c) \(\mathrm{LiClO}\).

Short answer

(a) Neutral, (b) Neutral, (c) Basic

Step-by-step solution

Identify the cation and anion in each salt

For each salt, list the cation (positive ion) and the anion (negative ion). (a) \(\text{(NH}_4\text{)}_3\text{PO}_4\): Cation: \(\text{NH}_4^+\), Anion: \(\text{PO}_4^{3-}\) (b) \(\text{Na}_2\text{SO}_4\): Cation: \(\text{Na}^+\), Anion: \(\text{SO}_4^{2-}\) (c) \(\text{LiClO}\): Cation: \(\text{Li}^+\), Anion: \(\text{ClO}^-\)

Determine the parent acid and base

Each salt is derived from a combination of a parent acid and a parent base. Determine these for each salt. (a) \(\text{(NH}_4\text{)}_3\text{PO}_4\): From \(\text{NH}_4^+\) (parent acid \(\text{NH}_4\text{OH}\)) and \(\text{PO}_4^{3-}\) (parent base \(\text{H}_3\text{PO}_4\)) (b) \(\text{Na}_2\text{SO}_4\): From \(\text{Na}^+\) (parent base \(\text{NaOH}\)) and \(\text{SO}_4^{2-}\) (parent acid \(\text{H}_2\text{SO}_4\)) (c) \(\text{LiClO}\): From \(\text{Li}^+\) (parent base \(\text{LiOH}\)) and \(\text{ClO}^-\) (parent acid \(\text{HClO}\))

Evaluate the strength of parent acid and base

Analyze the relative strengths of the parent acids and bases. (a) \(\text{(NH}_4\text{)}_3\text{PO}_4\): NH4OH is a weak base, and H3PO4 is a weak acid. (b) \(\text{Na}_2\text{SO}_4\): NaOH is a strong base, and H2SO4 is a strong acid. (c) \(\text{LiClO}\): LiOH is a strong base, and HClO is a weak acid.

Determine the resulting pH

Based on the strengths of the parent acid and base, determine whether the salt solution is acidic, basic, or neutral. (a) \(\text{(NH}_4\text{)}_3\text{PO}_4\): A weak base and a weak acid generally yield a neutral or slightly acidic solution, but the solution here is likely neutral because \(\text{NH}_4\text{OH}\) and \(\text{H}_3\text{PO}_4\) have approximately similar strengths. (b) \(\text{Na}_2\text{SO}_4\): A strong base and a strong acid will typically yield a neutral solution. (c) \(\text{LiClO}\): A strong base and a weak acid will result in a basic solution.

Key concepts

acid-base chemistry

In acid-base chemistry, acids are substances that can donate a proton (H+), while bases are substances that can accept a proton. When acids and bases react, they form salts. The nature of these salts in water depends on the strengths of the parent acids and bases from which they are derived. If the parent acid and base are both strong, the resulting salt solution will be neutral. Conversely, if a weak acid or base is involved, the solution can be acidic or basic. To determine the nature of a salt solution, we often consider the dissociation of the salt in water and the behaviors of the resulting ions.

salt hydrolysis

Salt hydrolysis refers to the reaction of salt ions with water to produce either hydronium ions (H3O+) or hydroxide ions (OH-). Depending on the ions produced, the solution can be acidic, basic, or neutral.
For example:

• Salts from strong acids and strong bases do not hydrolyze and thus create neutral solutions.
• Salts from strong bases and weak acids hydrolyze to yield basic solutions because the anion from the weak acid will react with water to form hydroxide ions.
• Salts from weak bases and strong acids hydrolyze to produce acidic solutions as the cation from the weak base will react with water to form hydronium ions.

parent acid and base

When we talk about salts, we often refer to their parent acid and base. These are the acid and base from which the salt is formed by neutralization.
For instance:

• In \((NH_4)_3PO_4\), \NH_4^+\ comes from NH_4OH (a weak base), and \PO_4^{3-}\ comes from \H_3PO_4\ (a weak acid).
• In \Na_2SO_4\, \Na^+\ is from NaOH (a strong base), and SO_4^{2-} is from H_2SO_4 (a strong acid).
• In \LiClO\, \Li^+\ originates from LiOH (a strong base) and ClO^- comes from HClO (a weak acid).

By analyzing the strengths of these parent compounds, we can predict the pH of the resulting salt solution.

solution pH

Solution pH measures the acidity or basicity of a solution. It is determined by the concentration of hydrogen ions (H+) in the solution.

• A pH less than 7 indicates an acidic solution.
• A pH of exactly 7 is neutral.
• A pH greater than 7 indicates a basic solution.

In the case of salts, the solution's pH is influenced by the hydrolysis of the ions.
For example:

• In \((NH_4)_3PO_4\), the solution is typically neutral because both the parent acid (H_3PO_4) and base (NH_4OH) are weak.
• In \Na_2SO_4\, the pH remains neutral due to the strong parent acid (H_2SO_4) and strong base (NaOH).
• In \LiClO\, the pH is basic because LiOH is a strong base, while HClO is a weak acid.