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 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 (b) \( \mathrm{CH}_{3} \mathrm{COONa} \).

Step 1: Understand the Problem

We need to determine which salt from the given options, when dissolved in water, creates a colourless solution with a pH of about 9. A pH of 9 indicates that the solution is weakly basic.

Step 2: Evaluate Each Salt

Let's evaluate each salt to identify which one would result in a basic solution: - (a)  ext{CaCO}_{3} : Calcium carbonate is sparingly soluble in water, and it would not readily form a solution with pH 9. It tends to form a slightly basic solution, but its limited solubility excludes it as an option. - (b)  ext{CH}_{3} ext{COONa} : Sodium acetate ionizes completely, forming acetate ions, which can react with water to produce hydroxide ions, generating a weakly basic solution. This will result in a pH around 9. - (c)  ext{NH}_{4} ext{NO}_{3} : Ammonium nitrate dissociates to form  ext{NH}_{4}^{+}, a weak acid, which typically results in an acidic solution rather than basic. - (d)  ext{CH}_{3} ext{COONH}_{4} : Ammonium acetate is neutral as both the acid and base components react, creating no significant shift in pH, around 7.

Step 3: Identify the Correct Salt

From Step 2, option (b)  ext{CH}_{3} ext{COONa}  is the salt that dissolves in water to form a solution with a pH of about 9, making it a weakly basic solution due to the presence of acetate ions.

Key Concepts

Basic Solutions

A solution is considered basic if its pH is greater than 7. The pH scale, which ranges from 0 to 14, measures the acidity or alkalinity of a solution.
If the pH is 7, the solution is neutral. If it's above 7, it's basic.
A pH of 9 indicates a weakly basic solution. This occurs when a salt dissolves in water and produces more hydroxide ions ( ext{OH}^-) than hydronium ions ( ext{H}_3 ext{O}^+).

In the case of sodium acetate ( ext{CH}_3 ext{COONa}), when dissolved in water, the acetate ion ( ext{CH}_3 ext{COO}^-) can interact with water molecules.
This reaction leads to the formation of acetic acid ( ext{CH}_3 ext{COOH}) and hydroxide ions.
The presence of these extra hydroxide ions is what shifts the pH to around 9, making the solution basic.

Solubility and Solubility Rules

Solubility describes how well a substance, such as a salt, can dissolve in a solvent like water. A highly soluble salt will readily dissolve, forming an aqueous solution.
When evaluating solubility, certain rules can help predict if a salt will dissolve.

For example:

• Nitrates ( ext{NO}_3^-), acetates ( ext{CH}_3 ext{COO}^-), and most alkali metal salts are generally soluble in water.
• Carbonates ( ext{CO}_3^{2-}) are usually insoluble, with exceptions like sodium carbonate ( ext{Na}_2 ext{CO}_3).

In our exercise, sodium acetate ( ext{CH}_3 ext{COONa}) is soluble and easily forms a clear solution.
Meanwhile, calcium carbonate ( ext{CaCO}_3) is less soluble and doesn't fit the criteria for the provided solution conditions.

Ionic Equilibrium

Ionic equilibrium refers to the state in a solution where ions are in balance, meaning the rates of forward and reverse reactions of ions are equal.
This balance is critical in understanding how acidic or basic a solution can become.

When sodium acetate ( ext{CH}_3 ext{COONa}) dissolves, it separates into ext{CH}_3 ext{COO}^- and ext{Na}^+ ions.
The ext{CH}_3 ext{COO}^- ion can then undergo hydrolysis with water, forming ext{CH}_3 ext{COOH} and ext{OH}^-.
In such systems, the concentration of hydroxide ions generated is balanced by the slightly ionized acetic acid that forms.
This balance helps maintain the pH around 9, signifying a weak base.
Such equilibria are key to predicting the behavior of salts in water.

Weak Acids and Bases

Weak acids and bases do not completely ionize in solutions, which means they partially disassociate into ions.
Acetic acid ( ext{CH}_3 ext{COOH}) is an example of a weak acid.

Unlike strong acids and bases, the incomplete ionization in weak acids and bases impacts the pH differently, often leading to a pH that is closer to neutral.
For acetic acid and acetate, this incomplete ionization results in a basic solution.
When ext{CH}_3 ext{COONa} dissolves, acetic acid forms but doesn't fully dissociate, keeping equilibrium in such a way that the solution remains weakly basic.
Understanding weak acids and bases is crucial for predicting the resulting pH when different compounds are dissolved in water.