Question

Arrange the following \(0.10 \mathrm{M}\) solutions in order of increasing acidity (decreasing \(\mathrm{pH}\) ): (i) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\), (ii) \(\mathrm{NaNO}_{3}\), (iii) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\), (iv) \(\mathrm{NaF}\), (v) \(\mathrm{CH}_{3} \mathrm{COONa}_{\text {. }}\)

Short answer

The given solutions can be arranged in order of increasing acidity (decreasing pH) as follows: 1. \(\mathrm{NaF}\) (most basic, highest pH) 2. \(\mathrm{CH}_{3} \mathrm{COONa}\) 3. \(\mathrm{NaNO}_{3}\) (neutral, pH 7) 4. \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (neutral to slightly acidic, close to pH 7) 5. \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (most acidic, lowest pH)

Step-by-step solution

Determine if each solution is an acid, base, or salt

(i) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\): This is a salt formed from the reaction of a weak base \(\mathrm{NH}_{3}\) and a strong acid \(\mathrm{HNO}_{3}\). It can hydrolyze in water. (ii) \(\mathrm{NaNO}_{3}\): This is a salt formed from the reaction of a strong base \(\mathrm{NaOH}\) and a strong acid \(\mathrm{HNO}_{3}\). It does not hydrolyze in water as both its ions are from strong acid and base. (iii) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\): This is a salt formed from the reaction of a weak acid \(\mathrm{CH}_{3} \mathrm{COOH}\) and a weak base \(\mathrm{NH}_{3}\). It can hydrolyze in water. (iv) \(\mathrm{NaF}\): This is a salt formed from the reaction of a strong base \(\mathrm{NaOH}\) and a weak acid \(\mathrm{HF}\). It can hydrolyze in water. (v) \(\mathrm{CH}_{3} \mathrm{COONa}_{\text {. }}\): This is a salt formed from the reaction of a weak acid \(\mathrm{CH}_{3} \mathrm{COOH}\) and a strong base \(\mathrm{NaOH}\). It can hydrolyze in water.

Determine the effect of each solution on pH

(i) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\): Since it is formed from a weak base and strong acid, it will hydrolyze to form \(\mathrm{NH}_{3}\) and \(\mathrm{HNO}_{3}\). The \(\mathrm{NH}_{3}\) formed is a weak base, and the \(\mathrm{HNO}_{3}\) is a strong acid. Thus, the solution will be acidic with a pH lower than 7. (ii) \(\mathrm{NaNO}_{3}\): Since it is formed from a strong acid and a strong base, it will not hydrolyze, and its pH will be neutral (7). (iii) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\): Since it is formed from a weak acid and a weak base, it will hydrolyze to form \(\mathrm{CH}_{3} \mathrm{COOH}\) and \(\mathrm{NH}_{3}\). The \(\mathrm{NH}_{3}\) formed is a weak base, and the \(\mathrm{CH}_{3} \mathrm{COOH}\) is a weak acid. The pH of the solution will depend on the relative strengths of the acid and base. In this case, it will be neutral to slightly acidic, close to pH 7. (iv) \(\mathrm{NaF}\): Since it is formed from a strong base and a weak acid, it will hydrolyze to form \(\mathrm{NaOH}\) and \(\mathrm{HF}\). The \(\mathrm{NaOH}\) formed is a strong base, while \(\mathrm{HF}\) is a weak acid. Thus, the solution will be basic with a pH higher than 7. (v) \(\mathrm{CH}_{3} \mathrm{COONa}_{\text {. }}\): Since it is formed from a weak acid and a strong base, it will hydrolyze to form \(\mathrm{CH}_{3} \mathrm{COOH}\) and \(\mathrm{NaOH}\). The \(\mathrm{CH}_{3} \mathrm{COOH}\) formed is a weak acid, while \(\mathrm{NaOH}\) is a strong base. Thus, the solution will be basic with a pH higher than 7.

Arrange solutions in order of increasing acidity (decreasing pH)

1. \(\mathrm{NaF}\) (most basic, highest pH) 2. \(\mathrm{CH}_{3} \mathrm{COONa}_{\text {. }}\) 3. \(\mathrm{NaNO}_{3}\) (neutral, pH 7) 4. \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (neutral to slightly acidic, close to pH 7) 5. \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (most acidic, lowest pH)

Key concepts

Hydrolysis of Salts

When a salt dissolves in water, it may undergo hydrolysis depending on the strength of its constituent acids and bases. Hydrolysis refers to the reaction of salt ions with water to form either acidic or basic solutions.

Salts derived from:

• Strong acid and strong base (e.g., NaNO₃): They do not undergo significant hydrolysis and usually result in a neutral solution, with a pH of around 7.
• Strong acid and weak base (e.g., NH₄NO₃): These salts will hydrolyze to produce an acidic solution. The weak base ion will combine with water to generate a small amount of hydroxide ions, making the solution acidic.
• Weak acid and strong base (e.g., NaF): These salts will hydrolyze to yield a basic solution. The weak acid ion combines with water, producing hydroxide ions, increasing the pH.
• Weak acid and weak base (e.g., CH₃COONH₄): The hydrolysis effect is more complex and depends on the relative strengths of the acid and base, making it neutral to slightly acidic or basic.

Understanding these categories helps predict the pH behavior of various salt solutions.

Weak Acids and Bases

Weak acids and bases do not completely dissociate in water, which influences the pH of their salts. Weak acids release fewer hydrogen ions ( H^+ ) in solution, while weak bases accept fewer hydrogen ions. This interplay is crucial in hydrolysis.

• Weak acids like acetic acid (CH₃COOH) produce anions that have a slight tendency to react with water. This forms hydroxide ions and makes the solution slightly basic when combined with strong bases.
• Weak bases such as ammonia (NH₃) result in cations that slightly react with water, donating protons and slightly increasing the hydrogen ion concentration. When these are combined with strong acids, the solution becomes acidic.

Weak acids and bases play a significant role in buffer solutions that resist drastic changes in pH.

Solution pH Determination

Determining the pH of a solution involves identifying the nature of the solute and its reaction in water. The dissociation of ions impacts the pH level significantly.

To determine pH:

• Evaluate the nature of the salt (acidic, basic, or neutral) by knowing the strengths of the acid and base that formed it.
• Hydrolysis reactions will inform whether the solution will lean towards acidic or basic.
• The net effect is the resultant pH, which tells us how acidic or basic the solution is compared to pure water.

For example:

• NaF solution will have a pH greater than 7 since it's formed from a weak acid and a strong base.
• NH₄NO₃ will have a pH less than 7 because it results from a strong acid and weak base.

Understanding these principles allows for precise predictions of pH levels in various solutions.