Question

The pH value of \(0.1 \mathrm{M}\) solutions of \(\begin{array}{llll}\mathrm{CH}_{3} \mathrm{COONa} & \text { (I), } & \mathrm{CH}_{3} \mathrm{COOH} & \text { (II), }\end{array}\) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (III), \(\mathrm{NaOH}\) (IV) and \(\mathrm{HCl}(\mathrm{V})\) is in the order (a) \(\mathrm{I}<\mathrm{II}<\mathrm{III}<\mathrm{IV}<\mathrm{V}\) (b) \(\mathrm{V}<\mathrm{IV}<\mathrm{III}<\mathrm{II}<\mathrm{I}\) (c) \(\mathrm{V}<\mathrm{II}<\mathrm{III}<\mathrm{I}<\mathrm{IV}\) (d) \(\mathrm{V}<\mathrm{II}<\mathrm{I}<\mathrm{III}<\mathrm{IV}\)

Short answer

The correct order of pH values from lowest to highest is HCl (V) < CH3COOH (II) < CH3COONH4 (III) < CH3COONa (I) < NaOH (IV), which corresponds to option (c).

Step-by-step solution

Identify the Nature of Each Solution

Determine if each substance (CH3COONa, CH3COOH, CH3COONH4, NaOH, and HCl) is an acid, base, or salt and how it affects the pH.

Analyze the Strength of Acids and Bases

CH3COOH is a weak acid, NaOH is a strong base, and HCl is a strong acid. CH3COONa is a salt deriving from a weak acid and a strong base, and CH3COONH4 is a salt deriving from a weak acid and a weak base.

Predict pH Values for Strong Acids and Bases

Strong acids and bases (HCl and NaOH) completely dissociate in solution, so HCl will have a very low pH, and NaOH will have a very high pH.

Assess the Effect of Salts on pH

CH3COONa will make the solution basic due to hydrolysis, since Na+ will not react with water but CH3COO- will. CH3COONH4 will have a slightly acidic pH because NH4+ will react with water to produce H+ ions, while the CH3COO- has only a minor basic effect.

Consider the Weak Acid

The weak acid (CH3COOH) will not completely dissociate and will result in a solution with pH less than 7 but higher than that of strong acid (HCl).

Arrange the Solutions in Order of Increasing pH

The pH values increase from the strongest acid to the strongest base as follows: HCl (V) with the lowest pH, then the weak acid CH3COOH (II), the salt from a weak base CH3COONH4 (III), the salt from a strong base CH3COONa (I), and finally the strong base NaOH (IV) with the highest pH.

Key concepts

Acid-Base Equilibrium

Understanding the concept of acid-base equilibrium is essential for predicting the behavior of acids, bases, and salts in solution. Acid-base equilibrium refers to the balance between the concentrations of acid (H⁺ donors) and base (OH^- acceptors) species in a solution. This balance is governed by the acid dissociation constant (K_a) for acids, and the base dissociation constant (K_b) for bases. The position of the equilibrium can tell us whether a given substance is a strong or weak acid/base.

For instance, strong acids like HCl, dissociate completely in water, shifting the equilibrium far to the right, thus releasing more H⁺ ions and resulting in a lower pH. Conversely, weak acids like CH₃COOH only partially dissociate, yielding fewer H⁺ ions, and therefore have a higher pH compared to strong acids. Appreciating these concepts helps students understand why the pH of HCl (V) is lower than that of CH₃COOH (II).

pH of Salts

The pH of salts arises from the hydrolysis reactions that occur when salts dissolve in water. Salts are ionic compounds that can affect pH based on the nature of their constituent ions. If a salt is composed of the anion from a weak acid and the cation from a strong base, such as CH₃COONa (I), the anion will undergo hydrolysis with water to form OH^- ions, resulting in a basic solution. On the other hand, salts like CH₃COONH₄ (III), formed from a weak acid and a weak base, can slightly tilt the pH to be more acidic because NH₄⁺ —a cation of a weak base—tends to donate H⁺ in the presence of water. This knowledge is crucial for accurately predicting the pH order as seen in the exercise. Salts can often be overlooked, but they play a significant role in defining the pH of a solution.

Strength of Acids and Bases

The strength of an acid or a base is a measure of its ability to donate or accept protons (H⁺ ions) when in solution. Strong acids, like HCl, and strong bases, like NaOH, completely dissociate into their ions in water, causing a significant change in the pH of their solutions. This is why HCl (V) produces a very low pH and NaOH (IV) yields a very high pH.

Weak acids and bases, such as CH₃COOH and NH₄OH, only dissociate partially, which means they do not change the pH of their solutions as dramatically. As mentioned in the exercise solution, CH₃COOH (II) is a weak acid and does not dissociate fully, leading to a moderately acidic pH. Acid and base strength is central to determining the pH level of a solution and is a key concept in any acid-base chemistry discussion.

Hydrolysis of Salts

Hydrolysis of salts is a chemical reaction involving the breaking of bonds in salts by water to form an acid or a base. The extent of this reaction affects the pH of the solution. For instance, in a salt such as CH₃COONa (I), derived from a weak acid (acetic acid) and a strong base (sodium hydroxide), the sodium ions (Na⁺) have no effect on the pH, whereas the acetate ions (CH₃COO^-) will react with water to produce OH^- ions, making the solution slightly basic.

Alternatively, when CH₃COONH₄ (III) dissolves, the ammonium ions (NH₄⁺) tend to donate H⁺ ions because NH₄⁺ is the conjugate acid of a weak base (ammonia), hence the solution becomes slightly acidic. Through hydrolysis, even neutral salts can influence the pH of a solution, which is an important concept in the context of acid-base reactions.