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) \(\mathrm{NaNO}_{3}\)

Short answer

Sodium carbonate (\( \mathrm{Na}_{2} \mathrm{CO}_{3} \)) has the highest pH.

Step-by-step solution

Identify the Nature of Salts

Determine whether each compound forms an acidic, basic, or neutral solution when dissolved in water.- (a) \( \text{CH}_3\text{COOK} \): Potassium acetate, a salt of a weak acid (acetic acid) and a strong base (potassium hydroxide), forms a basic solution.- (b) \( \text{Na}_2\text{CO}_3 \): Sodium carbonate, a salt of a weak acid (carbonic acid) and a strong base (sodium hydroxide), forms a basic solution.- (c) \( \text{NH}_4\text{Cl} \): Ammonium chloride, a salt of a weak base (ammonia) and a strong acid (hydrochloric acid), forms an acidic solution.- (d) \( \text{NaNO}_3 \): Sodium nitrate, a salt of a strong base (sodium hydroxide) and a strong acid (nitric acid), forms a neutral solution.

Compare Basic Salts

Compare the basicity of solutions formed by \( \text{CH}_3\text{COOK} \) and \( \text{Na}_2\text{CO}_3 \).- Sodium carbonate (\( \text{Na}_2\text{CO}_3 \)) is stronger as it forms carbonic acid (a weaker acid than acetic acid from \( \text{CH}_3\text{COOK} \)) and hydroxide ions, resulting in a higher pH compared to the acetate ion in potassium acetate.

Determine Highest pH

Since \( \text{Na}_2\text{CO}_3 \) forms the solution with the strongest basicity among the salts evaluated, it will have the highest pH when dissolved in pure water.

Key concepts

Acidic and Basic Salts

Salts can be classified as acidic, basic, or neutral based on the nature of the acids and bases from which they are formed. This classification is crucial in determining the pH level of a solution they form:

• Acidic salts: These are derived from a strong acid and a weak base. When dissolved in water, they typically lower the pH, creating an acidic solution. For instance, ammonium chloride (\( \text{NH}_4\text{Cl} \)) is an acidic salt resulting from ammonia (a weak base) and hydrochloric acid (a strong acid).
• Basic salts: Formed from a weak acid and a strong base, basic salts raise the pH of a solution. Potassium acetate (\( \text{CH}_3\text{COOK} \)) and sodium carbonate (\( \text{Na}_2\text{CO}_3 \)) are examples of salts that increase the pH level due to their basic nature.
• Neutral salts: These salts result from the reaction of a strong acid with a strong base and usually do not affect the pH of a solution significantly. Sodium nitrate (\( \text{NaNO}_3 \)) is such a salt and generally results in a neutral pH.

Understanding the nature of salts helps predict whether the resulting solution will be acidic, basic, or neutral, which is essential for various applications, including water treatment.

Water Chemistry

Water chemistry plays a pivotal role in various scientific and environmental processes. Water itself is a neutral molecule, but when salts are dissolved in it, the pH of the solution can change:
- When acidic salts are dissolved, water can exhibit acidic properties due to the production of excess hydrogen ions (\( \text{H}^+ \)). - Basic salts can lead to an increase in hydroxide ions (\( \text{OH}^- \)), raising the pH and making the solution alkaline.
To determine the acidity or basicity of a salt solution in water:

• Measure the pH: This indicates how acidic or basic a solution is. The pH scale ranges from 0 to 14, where 7 is neutral, less than 7 is acidic, and greater than 7 is basic.
• Identify the ionization process: Different salts ionize in water to produce different ions that affect pH. For example, sodium carbonate dissociates to produce carbonate ions, which then react with water to form bicarbonate and hydroxide ions, increasing pH.

Knowing these changes in water's chemistry is vital for fields such as agriculture, where soil pH impacts plant growth, and industry, where water treatment processes are optimized based on these principles.

Salt Hydrolysis

Salt hydrolysis is the reaction of salt ions with water. This reaction often results in the production of an acidic or basic solution:

• During hydrolysis, the ionized form of a salt interacts with water molecules. The process can lead to either a release of hydrogen ions (making the solution acidic) or hydroxide ions (making the solution basic).
• For basic salts: In the case of sodium carbonate, the carbonate ions can attract hydrogen ions from water, forming weak carbonic acid and releasing hydroxide ions, which increases the pH.
• For acidic salts: For a salt like ammonium chloride, the ammonium ion can release hydrogen ions into the water, making the solution acidic by reacting with water to form ammonia and increasing hydrogen ion concentration.

Understanding salt hydrolysis is essential because it determines the final pH of a solution and explains how weak acids or bases can influence a solution's properties when combined with water. This concept is fundamental in chemistry and can explain phenomena such as soil acidity, food preservation, and more.