Problem 42 - A Complete Resource Book in Chemistry for JEE Main

Question

The \(\mathrm{pH}\) of the neutralization point of \(0.1 \mathrm{~N}\) ammonium hydroxide with \(0.1 \mathrm{NHCl}\) is (a) 1 (b) 6 (c) 9 (d) 7

Step-by-Step Solution

Answer

The \( \text{pH} \) is approximately 6, which is option (b).

Step 1: Understanding the Neutralization Reaction

In this problem, we are dealing with the neutralization of ammonium hydroxide (a weak base) with HCl (a strong acid). The equation for the reaction is \( \text{NH}_4\text{OH} + \text{HCl} \rightarrow \text{NH}_4\text{Cl} + \text{H}_2\text{O} \). This forms ammonium chloride, which is slightly acidic in nature.

Step 2: Determine the Nature of the Salt

Ammonium chloride (\( \text{NH}_4\text{Cl} \)) is a salt formed by a weak base and a strong acid. The solution of \( \text{NH}_4\text{Cl} \) will be acidic. Thus, the \( \text{pH} \) of the solution will be less than 7.

Step 3: Calculate the pH at Neutralization

Since the reaction produces ammonium chloride, the solution tends to be acidic. Knowing that it's slightly acidic, the \( \text{pH} \) value is expected to be below 7. The most typical value for such a weak base and strong acid combination would be a \( \text{pH} \) of around 6.

Key Concepts

pH calculation

pH calculation is essential when analyzing the acidity or basicity of a solution. For the neutralization of ammonium hydroxide ( \(\text{NH}_4\text{OH}\)) with hydrochloric acid (HCl), we need to consider the resulting salt. Neutralization typically yields a salt and water. Let's break down the steps in pH calculation:

• Understand the nature of the reactants and products. Here, a weak base reacts with a strong acid.
• Recognize that the salt formed, ammonium chloride ( \(\text{NH}_4\text{Cl}\)), makes the solution slightly acidic.
• Use the properties of acids and bases to estimate pH, where a pH below 7 indicates acidity.

To determine the precise pH, one usually needs more detailed concentration data and may use the formula: \( \text{pH} = -\log[\text{H}^+] \). For a neutralization reaction like this, it's commonly found to be around 6, which indicates a slightly acidic solution.

acid-base reaction

In an acid-base reaction, an acid reacts with a base to form a salt and water. This is the essence of neutralization. With ammonium hydroxide ( \(\text{NH}_4\text{OH}\)) and hydrochloric acid (HCl), an interesting interaction occurs:

• The weak base, ammonium hydroxide, provides hydroxide ions (\(\text{OH}^-\)).
• Hydrochloric acid, a strong acid, provides hydrogen ions (\(\text{H}^+\)).
• The reaction forms water and a salt, ammonium chloride ( \(\text{NH}_4\text{Cl}\)).

This salt is important because it slightly shifts the pH due to the nature of its components. When a weak base and a strong acid react, the resulting solution is usually more acidic. This shift is an example of how varying strengths of reactants affect the outcome of neutralization reactions.

ammonium chloride

Ammonium chloride ( \(\text{NH}_4\text{Cl}\)) is the salt resulting from the neutralization of ammonium hydroxide and hydrochloric acid. Ammonium chloride is notable for being slightly acidic, which influences the solutions in which it is dissolved.
When ammonium chloride dissolves in water, it dissociates into ammonium ions (\(\text{NH}_4^+\)) and chloride ions (\(\text{Cl}^-\)).

• \(\text{NH}_4^+\) acts as a weak acid and can donate protons to the water, leading to a slight increase in the acidity of the solution.
• \(\text{Cl}^-\) does not contribute significantly to the pH as it is the conjugate base of a strong acid.

Due to this reaction, ammonium chloride solutions generally have a pH less than 7. This character makes ammonium chloride a useful compound for various applications such as a buffer in experiments or even in fertilizers. Understanding its properties helps in predicting the outcomes in chemical reactions and their practical applications.