Question

Mark the appropriate choice to fill up the blanks in the given paragraph. A solution which maintains constant \(\mathrm{pH}\) when small amounts of acid or base are added is known as a (i) A mixture of acetic acid and sodium acetate acts as (ii) with a pH around (iii) and a mixture of ammonium chloride and ammonium hydroxide acts as (iv) with a pH around \((v)\). (i) \(\begin{array}{lll}\text { (ii) } & \text { (iii) } & \text { (iv) }\end{array}\) (v) \(\begin{array}{lllll}\text { (a) buffer basic } & 9.25 & \text { acidic } & 4.75\end{array}\) capacity buffer buffer (b) buffer acidic \(9.25\) basic \(4.75\) solution buffer buffer \(\begin{array}{lllll}\text { (c) buffer basic } & 4.75 & \text { acidic } & 9.25\end{array}\) solution buffer buffer \(\begin{array}{lll}\text { (d) buffer acidic } & 4.75 \text { basic }\end{array}\) \(9.25\) solution buffer buffer

Short answer

(b) buffer, acidic, 4.75, basic, 9.25

Step-by-step solution

Identify what the question describes

The question describes a solution that can maintain a constant pH when small amounts of acid or base are added. This type of solution is known as a buffer solution. This will help us identify the correct term for the first blank.

Determine the correct term for the first blank (i)

The solution that maintains constant pH upon the addition of small quantities of acid or base is called a buffer. This will be the answer for blank (i).

Understand the actions of the given mixtures

The mixture of acetic acid and sodium acetate acts as a buffer that resists changes in pH. Similarly, the mixture of ammonium chloride and ammonium hydroxide also acts as a buffer. This helps us fill in blanks (ii) and (iv).

Determine the pH values of the given buffers and fill in blanks (iii) and (v)

Acetic acid is a weak acid and sodium acetate is its conjugate base, forming a buffer solution with a pH slightly acidic (less than 7). Ammonium chloride and ammonium hydroxide form a buffer solution with a pH slightly basic (greater than 7).

Select the correct statement from the options

Match our understanding from steps 2 and 4 with the given options to find that the correct answer uses 'buffer' and pairs acidic character with a pH lower than 7 and basic character with a pH higher than 7.

Key concepts

pH Maintenance

The concept of pH maintenance is essential in chemistry, particularly when dealing with solutions that must resist changes in pH when external substances are added. A buffer solution is the key to achieving this stability. By definition, a buffer solution is a system that minimizes changes in pH upon the addition of small amounts of an acid or a base.

These solutions contain a weak acid and its conjugate base or a weak base and its conjugate acid. When an acid (H+ donor) is added to the buffer, the conjugate base present in the buffer 'captures' the excess H+ ions, ensuring the pH remains relatively constant. Conversely, when a base (OH- donor) is added, the weak acid donates H+ ions to neutralize the OH- ions.

A real-world application of buffer solutions is seen in biological systems, where enzymes, which are sensitive to pH changes, require a stable environment to function optimally. Buffer solutions are also widely used in many industrial processes and in pharmaceutical products to ensure the stability of the pH.

Acetic Acid and Sodium Acetate Buffer

An acetic acid and sodium acetate buffer system is a common example of a buffer. Acetic acid (CH3COOH) is a weak acid, and sodium acetate (CH3COONa) provides the conjugate base (CH3COO-) in solution.

When small amounts of a strong acid are introduced to this buffer, the acetate ions combine with the hydrogen ions from the strong acid to form more acetic acid, thereby reducing the effect on the pH. Similarly, if a strong base is added, the acetic acid reacts with the hydroxide ions to form water and acetate ions, which also minimizes the pH change.

The pH of an acetic acid and sodium acetate buffer leans towards the acidic side because acetic acid only partially dissociates in solution. The pH is calculated using the Henderson-Hasselbalch equation, represented as \[ pH = pKa + \log\left(\frac{[A^-]}{[HA]}\right) \], where \(pKa\) is the acid dissociation constant of acetic acid, \([A^-]\) is the concentration of the acetate ion, and \([HA]\) is the concentration of acetic acid. It's crucial for students to grasp how the ratio of these components determines the solution's pH.

Ammonium Chloride and Ammonium Hydroxide Buffer

Another commonly discussed buffer system is composed of ammonium chloride (NH4Cl) and ammonium hydroxide (NH4OH). In this case, NH4Cl provides the weak acid (NH4+) and NH4OH provides the weak base (NH3).

When a strong acid is added to this buffer, the NH3 molecules neutralize the excess hydrogen ions by forming more NH4+ ions. Conversely, if a strong base is added, the NH4+ ions ionize to yield more NH3 and H+ ions, countering the base's effect on the pH.

The pH of this buffer system is typically above 7, making it slightly basic. However, this pH value is dependent on the relative concentrations of the ammonium ions and ammonia as dictated by the Henderson-Hasselbalch equation for bases: \[ pH = pKb + \log\left(\frac{[BH+]}{[B]}\right) \], where \(pKb\) is the base dissociation constant, \([BH+]\) is the concentration of the ammonium ion, and \([B]\) is the concentration of ammonia. Understanding how shifts in these concentrations influence the pH is vital for comprehending buffer action.