Problem 71 - Chemistry The Molecular Science

Question

For each salt, predict whether an aqueous solution has a pH less than, equal to, or greater than 7 . Explain your prediction. (a) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) (b) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}\) (c) \(\mathrm{KCH}_{3} \mathrm{COO}\)

Step-by-Step Solution

Answer

(a) Basic pH > 7; (b) Basic pH > 7; (c) Basic pH > 7.

Step 1: Identify Ions in the Salt

For each salt given, start by dissociating it into its constituent ions in water.(a) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) dissociates into two \(\mathrm{Na}^+\) ions and one \(\mathrm{HPO}_{4}^{2-}\) ion.(b) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}\) dissociates into two \(\mathrm{NH}_{4}^+\) ions and one \(\mathrm{S}^{2-}\) ion.(c) \(\mathrm{KCH}_{3} \mathrm{COO}\) dissociates into one \(\mathrm{K}^+\) ion and one \(\mathrm{CH}_{3}\mathrm{COO}^-\) ion.

Step 2: Analyze the Ions' Acid-Base Properties

Identify whether the ions are acidic, basic, or neutral in solution.(a) \(\mathrm{Na}^+\) is neutral, while \(\mathrm{HPO}_{4}^{2-}\) acts as a weak base because it can accept a proton to form \(\mathrm{H}_2 \mathrm{PO}_4^-\).(b) \(\mathrm{NH}_{4}^+\) is a weak acid, as it can donate a proton to form \(\mathrm{NH}_3\), while \(\mathrm{S}^{2-}\) is a strong base as it can accept two protons to form \(\mathrm{H}_2\mathrm{S}\).(c) \(\mathrm{K}^+\) is neutral, while \(\mathrm{CH}_{3}\mathrm{COO}^-\) is a weak base as it can accept a proton to form acetic acid \(\mathrm{CH}_{3}\mathrm{COOH}\).

Step 3: Determine the Net Effect on pH

For each salt, determine if the overall effect is acidic, basic, or neutral by considering the strengths of the acidic and basic ion components.(a) Since \(\mathrm{Na}^+\) is neutral and \(\mathrm{HPO}_{4}^{2-}\) is weakly basic, the solution is basic, thus \(\text{pH} > 7\).(b) Here, \(\mathrm{NH}_{4}^+\) is a weak acid and \(\mathrm{S}^{2-}\) is a much stronger base, so the solution is basic, thus \(\text{pH} > 7\).(c) With \(\mathrm{K}^+\) being neutral and \(\mathrm{CH}_{3}\mathrm{COO}^-\) being weakly basic, the overall solution is basic, thus \(\text{pH} > 7\).

Step 4: Conclusion

Based on the previous analysis, determine the expected pH for each salt when dissolved in water:(a) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) results in a basic solution.(b) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}\) results in a basic solution.(c) \(\mathrm{KCH}_{3} \mathrm{COO}\) results in a basic solution.

Key Concepts

Acid-Base Properties of Ions

When dissolved in water, salts dissociate into their respective cations and anions. The acid-base properties of these ions play a crucial role in determining the pH of the resulting aqueous solutions. Here's what you need to know:

• Neutral ions like sodium ion (\(\mathrm{Na}^+\)) or potassium ion (\(\mathrm{K}^+\)) do not affect the pH of the solution significantly, as they neither accept nor donate protons.
• Acidic ions, such as the ammonium ion (\(\mathrm{NH}_4^+\)), can donate protons to the solution, potentially lowering the pH if the associated base is significantly weaker.
• Basic ions, like the phosphate ion (\(\mathrm{HPO}_4^{2-}\)) or acetate ion (\(\mathrm{CH}_3\mathrm{COO}^-\)), accept protons, which can increase the pH.

The overall pH depends on the relative strengths of these acidic and basic ions. A basic ion in solution will generally make the solution basic, especially if the corresponding acidic ion is weak or neutral. Similarly, a strong acidic ion can counterbalance weak basic ions.

Aqueous Solutions

An aqueous solution is simply a solution where water acts as the solvent. When salts are introduced to water, they usually dissociate into ions even if they are initially solid. Understanding the nature of the aqueous solution is essential:

• Dissolution: Salts such as \(\mathrm{Na}_2\mathrm{HPO}_4\), \((\mathrm{NH}_4)_2\mathrm{S}\), and \(\mathrm{KCH}_3\mathrm{COO}\) break apart into neutral ions and possibly reactive ions. This is the first step in understanding how salts affect water's chemistry.
• pH Measurement: The pH of an aqueous solution directly reflects the concentration of hydrogen ions. The scale ranges from 0 to 14, where 7 is neutral, below 7 is acidic, and above 7 is basic. Given this, the dissociated ions' nature can shift the balance of hydrogen and hydroxide ions, altering the overall pH.

It's primarily the interaction of these ions in water that will determine whether the resulting solution is acidic or basic. The key lies in the solute's behavior in water, which stems from the chemical nature of its constituent ions.

Dissociation of Salts

The dissociation process of salts in water is essential to understanding chemical equilibria in solution. Each salt dissociates differently, producing varied ions that contribute to the solution's overall acidity or basicity:- For \(\mathrm{Na}_2\mathrm{HPO}_4\), dissociation results in sodium ions (\(\mathrm{Na}^+\)) and hydrogen phosphate ions (\(\mathrm{HPO}_4^{2-}\)).- In the case of \((\mathrm{NH}_4)_2\mathrm{S}\), it dissociates into ammonium ions (\(\mathrm{NH}_4^+\)) and sulfide ions (\(\mathrm{S}^{2-}\)).- Finally, \(\mathrm{KCH}_3\mathrm{COO}\) yields potassium ions (\(\mathrm{K}^+\)) and acetate ions (\(\mathrm{CH}_3\mathrm{COO}^-\)).The resulting ions from dissociation have unique properties. For instance, the \(\mathrm{HPO}_4^{2-}\) acts as a weak base, absorbing hydrogen ions, while \(\mathrm{NH}_4^+\) acts as a weak acid, releasing hydrogen ions. These interactions define the pH of the solution: whether it stays neutral, becomes acidic, or leans towards basic. Dissociating salts creates opportunities for these chemical processes to occur, ultimately affecting the environment within the solution.