Problem 26 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The \(\mathrm{pH}\) of \(0.1 \mathrm{M}\) solution of the following salts increases in the order. (a) \(\mathrm{NaCl}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCN}<\mathrm{HCl}\) (b) \(\mathrm{HCl}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCl}<\mathrm{NaCN}\) (c) \(\mathrm{NaCN}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCl}<\mathrm{HCl}\) (d) \(\mathrm{HCl}<\mathrm{NaCl}<\mathrm{NaCN}<\mathrm{NH}_{4} \mathrm{Cl}\)

Step-by-Step Solution

Answer

(b) \(\text{HCl} < \text{NH}_4\text{Cl} < \text{NaCl} < \text{NaCN}\)

Step 1: Understanding the Nature of Salts

First, identify the nature of each salt in the given options. \(\text{NaCl}\) is a neutral salt formed from a strong base (NaOH) and strong acid (HCl), \(\text{NH}_4\text{Cl}\) is an acidic salt formed from a weak base (NH₃) and a strong acid (HCl), and \(\text{NaCN}\) is a basic salt formed from a strong base (NaOH) and a weak acid (HCN). \(\text{HCl}\) is a strong acid.

Step 2: Determine pH Characteristics

Identify the pH characteristics of each salt. \(\text{NaCl}\) has a neutral pH around 7, \(\text{NH}_4\text{Cl}\) has an acidic pH since it results in the formation of \(\text{NH}_4^+\), which releases \(\text{H}^+\) ions, \(\text{NaCN}\) has a basic pH as it forms \(\text{CN}^-\), which can accept \(\text{H}^+\) ions, and \(\text{HCl}\) is a strong acid, giving it a very low pH.

Step 3: Arrange Based on pH from Low to High

With \(\text{HCl}\) having the lowest pH, followed by \(\text{NH}_4\text{Cl}\), then \(\text{NaCl}\), and finally \(\text{NaCN}\), which has the highest pH due to its basic nature, arrange them to reflect increasing pH.

Key Concepts

Acidic and Basic Salts

Salts are formed from the reaction between acids and bases. However, they don't all have the same pH level and can be broadly classified into acidic or basic salts. Acidic salts, like ammonium chloride (\(\text{NH}_4\text{Cl}\)), emerge from a weak base and a strong acid. This leads to more hydrogen ions (\(\text{H}^+\)), giving the solution an acidic pH below 7. Conversely, basic salts such as sodium cyanide (\(\text{NaCN}\)) are formed from strong bases and weak acids, contributing to a higher pH above 7, as the solution tends toward alkalinity. Understanding the formation helps to determine their effects on the pH level.

Neutral Salts

Neutral salts result from the combination of strong acids and strong bases, such as sodium chloride (\(\text{NaCl}\)). Unlike acidic or basic salts, neutral salts do not alter the concentration of \(\text{H}^+\) or hydroxide (\(\text{OH}^-\)) ions in a solution significantly. Thus, they typically have a pH level close to 7, which is consistent with neutral solutions. Key examples include table salt, which neither makes a solution more acidic nor basic. Know this type of salt to better predict the behavior of various solutions when they dissolve.

pH Scale

The pH scale is an essential tool in chemistry to measure the acidity or basicity of a solution. Ranging from 0 to 14, the scale is logarithmic. A pH of 7 is neutral, indicating an equal concentration of \(\text{H}^+\) and \(\text{OH}^-\) ions. Values below 7 are acidic, meaning there are more \(\text{H}^+\) ions compared to hydroxide ions. Values above 7 indicate a basic solution, with fewer \(\text{H}^+\) ions. To determine a solution's type and behavior, understanding where it falls on the pH scale is crucial. Practicing with it can make acid-base reactions much clearer.