Problem 17 - A Complete Resource Book in Chemistry for JEE Main

Question

A white salt is readily soluble in water and gives a colourless solution with a \(\mathrm{pH}\) of about \(9 .\) The salt could be (a) \(\mathrm{CaCO}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{COONa}\) (c) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\)

Step-by-Step Solution

Answer

The salt is \(\mathrm{CH}_{3} \mathrm{COONa}\) (sodium acetate).

Step 1: Understanding the Salt Solution

We are given a salt that is soluble in water, forming a colourless solution with a pH of about 9. A pH of 9 suggests a basic solution, indicating the presence of a weak acid and a strong base.

Step 2: Analyzing Options

Examine each salt to understand its components:(a) \(\mathrm{CaCO}_{3}\): Calcium carbonate is only slightly soluble in water and usually gives a neutral to slightly basic solution due to carbonate ions, but it doesn't form a clear solution.(b) \(\mathrm{CH}_{3} \mathrm{COONa}\): Sodium acetate undergoes hydrolysis in water, giving a basic solution as acetate ions react with water to form \(\mathrm{OH}^-\).(c) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\): Ammonium nitrate is acidic due to ammonia dissolution and nitrate equilibrium.(d) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\): Ammonium acetate forms a neutral solution since both acetate and ammonium ions balance each other out in terms of acidity and basicity.

Step 3: Selecting the Correct Salt

From the analysis in Step 2, \(\mathrm{CH}_{3} \mathrm{COONa}\) (sodium acetate) is the only salt that forms a basic solution with a pH of about 9. It releases acetate ions that undergo hydrolysis, increasing the pH by forming weak acetic acid and hydroxide ions.

Key Concepts

pH of Solutions

The concept of pH is fundamental in understanding whether a solution is acidic, neutral, or basic. It's a numeric scale used to specify the acidity or basicity of an aqueous solution. Importantly, the pH scale ranges from 0 to 14.
- A pH less than 7 indicates an acidic solution. - A pH of exactly 7 indicates neutrality, meaning the solution is neither acidic nor basic. - A pH greater than 7 indicates a basic or alkaline solution.
In the context of this exercise, the solution has a pH of about 9, which means it's basic. This suggests that the salt dissolved in water has components that increase the concentration of hydroxide ions \( ext{OH}^- \) in the solution, shifting the balance towards the basic end of the pH scale.

Salt Hydrolysis

Salt hydrolysis is a reaction where water and salts interact, leading to the formation of an acidic or basic solution. This process occurs when either the cation or anion (or both) of a dissolved salt reacts with water.
Let's break it down:- If the anion comes from a weak acid, it tends to react with water, generating OH\( ^- \) ions, making the solution basic.- Conversely, if the cation comes from a weak base, it may react with water to produce H\( ^+ \) ions, making the solution acidic.
In the exercise, sodium acetate (\( \text{CH}_3 \text{COONa} \)) illustrates this process. When sodium acetate dissolves, acetate ions (\( \text{CH}_3 \text{COO}^- \)) react with water to form acetic acid and hydroxide ions, giving the solution a basic characteristic.

Weak Acid-Strong Base Reaction

When we talk about a weak acid reacting with a strong base, it's important to understand the nature of these chemical reactions. In these scenarios:- A **weak acid** doesn't completely dissociate in water, meaning not all of its molecules release hydrogen ions \( ext{H}^+ \). This results in partially ionized acids.- A **strong base**, on the other hand, dissociates fully in water, releasing OH\( ^- \) ions into the solution efficiently.
In the case of sodium acetate mentioned in the exercise, the acetate ion from acetic acid (a weak acid) performs hydrolysis, interacting with water. Sodium, however, comes from sodium hydroxide (a strong base), which fully dissociates in water. The basic nature of acetate in the presence of sodium ions results in a solution that is basic, evidenced by a higher pH of 9 in this case.