Question

From separate solutions of four sodium salts NaW, NaX, NaY and \(\mathrm{NaZ}\) had \(\mathrm{pH} 7.0,9.0,10.0\) and \(11.0\) respectively, when each solution was \(0.1 M\), the weakest acid is : (a) HW (b) \(\mathrm{HX}\) (c) HY. (d) \(\mathbf{H Z}\)

Short answer

The weakest acid is (d) HZ.

Step-by-step solution

Understanding pH

The pH scale measures the acidity or basicity of a solution. A pH of 7 is considered neutral, pH levels lower than 7 indicate acidity, and pH levels higher than 7 indicate basicity. As pH value increases, the acidity decreases, meaning the substance is more basic (or less acidic).

Finding the Basicity

Given the pH values of the solutions of NaW, NaX, NaY, and NaZ, we can infer that the higher the pH, the more basic (or less acidic) the solution. Thus the salt that produces the solution with the highest pH will be the one derived from the weakest acid.

Determine the Weakest Acid

Comparing the given pH values: pH 7 (neutral), pH 9, pH 10, and pH 11, the salt NaZ which has a pH of 11 is the most basic solution. Therefore, HZ is derived from the weakest acid because its salt forms the most basic solution.

Key concepts

pH Scale

Understanding the pH scale is fundamental when studying chemistry, especially when it comes to understanding acidity and basicity of solutions. The pH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It ranges from 0 to 14, with 7 being neutral. A pH less than 7 indicates an acidic solution, and a pH greater than 7 indicates a basic (alkaline) solution. The scale is based on the concentration of hydrogen ions ((\[ H^+ \])) present in a solution.

For instance, with a pH of 7, water is neutral, meaning it's neither acidic nor basic. In the exercise provided, when the sodium salts have pH values above 7, it indicates that their corresponding acids must be weak, as weak acids dissociate less in water and produce less hydrogen ions. This explains why the salt NaZ, which produced a solution with a pH of 11, indicated that HZ is the weakest acid among the ones considered.

Acid-Base Chemistry

In acid-base chemistry, acids and bases interact with each other forming water and salts. Acids are substances that can donate a hydrogen ion ((\[ H^+ \])) whereas bases are substances that can accept (\[ H^+ \])) ions. This interaction is the classic Bronsted-Lowry definition of acids and bases. The strength of an acid or base is typically measured by the extent to which it dissociates in solution to form ions.

A strong acid or base will completely dissociate, while a weak acid or base will only partially dissociate. Using the exercise as an example, the acid HZ would be classified as a weak acid, as it has a higher pH value when in the form of its sodium salt ((\[ NaZ \])) solution, indicating that it does not fully dissociate and hence its (\[ H^+ \])) ions are less available in the solution.

Chemical Equilibrium

Chemical equilibrium occurs when the rates of forward and reverse chemical reactions are equal, resulting in no net change in the amount of reactants and products. This is a dynamic state where the conversion of reactants into products and the conversion of products back into reactants occur simultaneously at the same rate.

In the context of acids and their salts, like in the exercise, an acid dissociates into its ions. Equilibrium is achieved when the rate of dissociation of the acid into ions equals the rate of recombination of the ions into the acid. The strength of an acid is related to its equilibrium position; a weak acid like HZ will favor the undissociated form, while a strong acid will favor ion formation. Equilibrium plays a vital role in understanding the behavior of acids and bases in solution.

Ionic Equilibrium in Solutions

Ionic equilibrium refers to the state of balance between the dissociated ions and the undissociated molecules of an electrolyte in solution. It is a specific type of chemical equilibrium dealing with the ionization of acids, bases, and salts. Ionic equilibrium is essential for understanding the pH and the strength of acids and bases.

The extent to which a salt dissociates into its constituent ions in solution depends on the nature of the acid and base from which the salt is derived. As illustrated by NaZ in the exercise, which has a high pH indicating a basic solution, the ionic equilibrium lies far to the right, meaning that the acid HZ is less apt to donate (\[ H^+ \])) ions due to its weak nature. The concept of ionic equilibrium is central to predicting the behavior of ions in solution, which directly affects the pH of the solution.