Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Which has highest \(\mathrm{pH}\) ? (a) \(\mathrm{CH}_{3} \mathrm{COOK}\) (b) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (c) \(\mathrm{NH}_{4} \mathrm{Cl}\) (d) \(\mathrm{NaNO}_{3}\)

Step-by-Step Solution

Answer

The highest pH is for Na_{2}CO_{3} because of the strong base CO_{3}^{2-}.

Step 1: Understanding pH

pH is a measure of the hydrogen ion concentration in a solution. A high pH value corresponds to a low concentration of hydrogen ions, thus indicating a basic or alkaline solution.

Step 2: Dissociation of Salts

Analyze how each salt dissociates in water: - (a) CH_{3}COOK  dissociates into CH_{3}COO^- and K^+ ions. The acetate ion  CH_{3}COO^- forms a basic solution. - (b) Na_{2}CO_{3}  dissociates into 2Na^+ and CO_{3}^{2-} ions. The carbonate ion  CO_{3}^{2-} is strongly basic. - (c) NH_{4}Cl  dissociates into NH_{4}^+ and Cl^- ions. Ammonium  NH_{4}^+ is acidic. - (d) NaNO_{3}  dissociates into Na^+ and NO_{3}^- ions. Both ions are neutral with respect to pH.

Step 3: Compare Basicity of Ions

Identify the basicity or acidity of the ions: - Carbonate  CO_{3}^{2-} is a strong base. - Acetate  CH_{3}COO^- is a weak base. - Ammonium  NH_{4}^+ is a weak acid. - Nitrate  NO_{3}^- and Na^+ are neutral in this context. Thus, salts like NaNO_{3} are neutral.

Step 4: Conclusion

Since Na_{2}CO_{3} dissociates to produce a strong base CO_{3}^{2-}, it results in the highest pH compared to the other options, given the strong basic nature of the carbonate ion.

Key Concepts

Dissociation of Salts

When salts dissolve in water, they dissociate into ions. This process is crucial to understanding the properties of the resulting solution.

• CH₃COOK dissociates into CH₃COO^- and K⁺. The acetate ion, CH₃COO^-, is essentially responsible for influencing the pH through its basic nature.
• Na₂CO₃ dissociates into 2 Na⁺ and CO₃^2-. The carbonate ion, CO₃^2-, is a very strong base, significantly raising the pH.
• NH₄Cl splits into NH₄⁺ and Cl^-. Here, NH₄⁺ is acidic, which lowers the pH.
• NaNO₃ results in Na⁺ and NO₃^-, where both ions do not affect the pH, maintaining neutrality.

When examining dissociation, it's important to note the influence of the resulting ions on the acidity or basicity of the solution. Basic ions increase pH, while acidic ions decrease it.

Basic and Acidic Ions

Ions can be categorized based on their ability to increase or decrease the pH of a solution. Each salt dissociates into ions, and these ions retain characteristics that define whether they are basic, acidic, or neutral.

• Basic ions: For example, the carbonate ion, CO₃^2-, is strongly basic. This means it can significantly increase the pH of a solution by decreasing the concentration of hydrogen ions.
• Acidic ions: An example is the ammonium ion, NH₄⁺. This ion tends to increase the concentration of hydrogen ions, thus lowering the pH.
• Neutral ions: Nitrate ions, NO₃^-, and sodium ions, Na⁺, do not typically alter the pH of a solution. They are considered neutral because they neither donate nor accept hydrogen ions.

Understanding the nature of these ions helps predict how a solution will behave once a salt is dissolved in it. Ions that are basic or acidic can shift the equilibrium of hydrogen ions, directly affecting the solution's pH.

Carbonate Ion Basicity

The carbonate ion (CO₃^2-) plays a critical role in determining the pH of solutions where it is present. It is known as a strong base, meaning it is very efficient at increasing pH levels.

• The chemistry behind this lies in the carbonate ion's ability to accept hydrogen ions. This decreases the overall hydrogen ion concentration in the solution, leading to a higher pH value.
• In practical terms, when Na₂CO₃ is placed in water, its dissociation releases carbonate ions that actively work to neutralize any acidic components, pushing the solution's environment towards a more basic state.

By understanding the basicity of the carbonate ion, one can predict that solutions containing significant amounts of CO₃^2- will tend to have a higher pH. This is why in the original exercise, Na₂CO₃ is noted to have the highest pH compared to other salts.