Problem 6 - Chemistry in 30 days

Question

The \(\mathrm{pH}\) of \(0.1 \mathrm{M}\) solution of the following salts increases in the order of (a) \(\mathrm{NaCl}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCN}<\mathrm{HCl}\) (b) \(\mathrm{HCl}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCl}<\mathrm{NaCN}\) (c) \(\mathrm{NaCN}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCl}<\mathrm{HCl}\) (d) \(\mathrm{HCl}<\mathrm{NaCl}<\mathrm{NaCN}<\mathrm{NH}_{4} \mathrm{Cl}\)

Step-by-Step Solution

Answer

(b) HCl < NH₄Cl < NaCl < NaCN

Step 1: Understand the Salt and Its Hydrolysis

To solve this problem, we need to understand the behavior of these salts in water. Salts can undergo hydrolysis, which affects the pH of their solutions. NaCl is a neutral salt, which means it will not affect the pH significantly. NaCN is a basic salt because it forms a basic solution by hydrolyzing to form CN⁻ which reacts with water to form OH⁻. NH₄Cl is an acidic salt because it hydrolyzes to form NH₄⁺ which creates H⁺ ions in solution. HCl is a strong acid, and in solution, it completely dissociates, significantly lowering the pH.

Step 2: Analyze pH Effect of Each Salt

HCl being a strong acid will have the lowest pH in solution since it dissociates completely. NH₄Cl is an acidic salt, so it will have a slightly higher pH than HCl. NaCl, being neutral, will have a pH close to 7, which is higher than that of NH₄Cl. NaCN being a basic salt will have the highest pH among the given salts due to the formation of OH⁻ ions in solution.

Step 3: Order pH from Lowest to Highest

Based on the hydrolysis behavior, arrange the salts in order of increasing pH. Starting from the lowest pH, we have HCl, followed by NH₄Cl (acidic salt), then NaCl (neutral), and finally NaCN (basic salt). This gives us the order: HCl < NH₄Cl < NaCl < NaCN.

Key Concepts

Salt Hydrolysis

When salts are dissolved in water, they can undergo a process known as salt hydrolysis. This means that the ions in the salt can interact with the water to produce acidic or basic solutions.

• Hydrolysis affects the pH of the solution, which is a measure of how acidic or basic the solution is.
• Not all salts undergo significant hydrolysis. The nature of the cation or anion derived from the salt determines if hydrolysis occurs and what type of ions are produced.

Understanding the principles of salt hydrolysis is essential when predicting the pH of a solution.

Acidic and Basic Salts

Salts are formed from the reaction of acids and bases. Depending on the strength and nature of the reactants, they can be classified as acidic, basic, or neutral.
An acidic salt is formed when a strong acid reacts with a weak base. The resulting salt can hydrolyze to produce H⁺ ions, thus lowering the pH of a solution. For example, ammonium chloride (\(\text{NH}_4\text{Cl}\)) is an acidic salt because it contains the ammonium ion (\(\text{NH}_4^+\)) that hydrolyzes to release H⁺.
Basic salts result from the reaction of a strong base with a weak acid. These salts produce OH⁻ ions when they hydrolyze, which increases the pH and makes the solution basic. An example is sodium cyanide (\(\text{NaCN}\)), which hydrolyzes to yield CN⁻ that reacts with water to form OH⁻.
Identifying whether a salt is acidic or basic helps predict how it will affect the solution's pH.

Strong Acids

Strong acids are characterized by their complete dissociation in water. This means they release all their hydrogen ions into the solution.

• This complete dissociation leads to a very low pH, indicating that the solution is highly acidic.
• Common examples of strong acids include hydrochloric acid (\(\text{HCl}\)), sulfuric acid (\(\text{H}_2\text{SO}_4}\)), and nitric acid (\(\text{HNO}_3\)).

For instance, when \(\text{HCl}\) is dissolved in water, it dissociates completely into H⁺ and Cl⁻ ions, causing the solution to exhibit a very low pH.

Neutral Salts

Neutral salts derive from the reaction between strong acids and strong bases. When these salts dissolve in water, they do not affect the pH significantly compared to acidic or basic salts.
Sodium chloride (\(\text{NaCl}\)) is a classic example of a neutral salt. It is formed from the reaction of hydrochloric acid (\(\text{HCl}\)) and sodium hydroxide (\(\text{NaOH}\)).

• When \(\text{NaCl}\) dissolves, it neither releases H⁺ nor OH⁻, so the pH remains neutral, around 7.
• This property is because neither of its ions, Na⁺ nor Cl⁻, reacts significantly with water.

Understanding neutral salts is important because they help maintain the pH balance in solutions.