Question

Which of the following statements is wrong? (1) for an acid buffer solution the \(\mathrm{pH}\) can be increased by inereasing the concentration of salt (2) for a basic buffer solution the pH can increased by decreasing the concentration of base(3) buffer capacity of a buffer solution is maximum when the ratio of salt/acid or base is 1 (4) the number of moles of acid or base required by one litre of buffer solution to alter its \(\mathrm{pH}\) by one unit is called buffer efficiency

Short answer

Statement 2 is incorrect.

Step-by-step solution

Understand Buffer Solutions

Buffer solutions resist changes in \(\text{pH}\) upon the addition of small amounts of acids or bases. They are composed of a weak acid and its conjugate base or a weak base and its conjugate acid.

Evaluate Statement 1

Statement 1 says that for an acid buffer solution, the \(\text{pH}\) can be increased by increasing the concentration of salt. This is correct. For an acid buffer, raising the amount of the conjugate base (salt), increases \(\text{pH}\).

Evaluate Statement 2

Statement 2 says that for a basic buffer solution, the \(\text{pH}\) can be increased by decreasing the concentration of base. This is incorrect. Reducing the concentration of the base would typically lower \(\text{pH}\), not increase it.

Evaluate Statement 3

Statement 3 mentions that the buffer capacity is maximum when the ratio of salt to acid or base is 1. This is correct as the buffer capacity is highest in this scenario.

Evaluate Statement 4

Statement 4 defines buffer efficiency as the number of moles of acid or base required by one litre of buffer solution to alter its \(\text{pH}\) by one unit. This definition is correct.

Identify the Incorrect Statement

Based on the evaluations, Statement 2 is determined to be incorrect.

Key concepts

Acid Buffer

An acid buffer is a solution that resists changes in its \(\text{pH}\) when small amounts of an acid or base are added. Typically, an acid buffer consists of a weak acid and its salt (which is the conjugate base of the weak acid).

For example, a common acid buffer is made from acetic acid (\text{CH}_3\text{COOH}) and sodium acetate (\text{CH}_3\text{COONa}).

In the buffer, the acetic acid can neutralize any added base, while the sodium acetate can neutralize any added acid. This helps maintain a stable \(\text{pH}\).

By increasing the concentration of the salt (sodium acetate in this example), the \(\text{pH}\) of the acid buffer can be increased. This is because the additional conjugate base can react with more hydrogen ions, reducing their concentration and thus increasing the \(\text{pH}\).

Basic Buffer

A basic buffer is a solution that resists changes in its \(\text{pH}\) when small amounts of an acid or base are added. It typically consists of a weak base and its salt (which is the conjugate acid of the weak base).

A classic example of a basic buffer is made from ammonia (\text{NH}_3) and ammonium chloride (\text{NH}_4\text{Cl}).

In this buffer, ammonia can neutralize any added acid, while ammonium ions (\text{NH}_4^+) can neutralize any added base. This stabilizes the \(\text{pH}\).

However, it is important to note that decreasing the concentration of the base in a basic buffer will not increase the \(\text{pH}\). In reality, reducing the amount of the weak base usually lowers the \(\text{pH}\) was there are fewer hydroxyl ions (\text{OH}^-) to neutralize added acids.

Buffer Capacity

Buffer capacity refers to the amount of acid or base a buffer solution can absorb without a significant change in \(\text{pH}\).

The buffer capacity is at its maximum when the ratio of the concentrations of the salt to the acid (for acid buffers) or the salt to the base (for basic buffers) is 1:1.

This is because, at this ratio, the buffer has equal capacities to neutralize both added acids and bases. As the concentrations diverge from this ratio, the buffer’s ability to resist changes in \(\text{pH}\) decreases.

For practical purposes, the buffer capacity is highly relevant in processes such as chemical reactions, biological systems, and industrial applications where maintaining a stable \(\text{pH}\) is crucial.

pH

The \(\text{pH}\) is a measure of the hydrogen ion concentration in a solution. It indicates how acidic or basic the solution is.

The \(\text{pH}\) scale ranges from 0 to 14:
• A \(\text{pH}\) less than 7 indicates an acidic solution.
• A \(\text{pH}\) of 7 indicates a neutral solution.
• A \(\text{pH}\) greater than 7 indicates a basic (alkaline) solution.

Mathematically, \(\text{pH}\) is defined as:
\[ \text{pH} = -\text{log} [\text{H}^+] \]
where \([\text{H}^+]\) is the concentration of hydrogen ions in the solution.

Understanding \(\text{pH}\) is crucial for working with buffer solutions, as the primary purpose of a buffer is to maintain a desired \(\text{pH}\) range in the face of external changes. Effective buffers are chosen based on how closely their pKa values (the pH at which half of the buffer is dissociated) match the desired \(\text{pH}\) of the solution.