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 sodium acetate (\( CH_3COONa \)).

Step 1: Analyzing the pH Requirement

The problem states the solution has a pH of about 9. This indicates the solution is basic. To achieve this, the salt must be made of a weak acid and a strong base.

Step 2: Evaluating \\( CaCO_3 \\\)

Calcium carbonate (\( CaCO_3 \)) is not readily soluble in water, and if it were to dissolve, the solution would likely have a neutral or slightly basic pH. However, it does not adequately explain a pH of 9.

Step 3: Evaluating \\( CH_3COONa \\\)

Sodium acetate (\( CH_3COONa \)) is a salt of a weak acid (acetic acid) and a strong base (sodium hydroxide). When dissolved, it forms a basic solution, aligning with a pH of around 9.

Step 4: Evaluating \\( NH_4NO_3 \\\)

Ammonium nitrate (\( NH_4NO_3 \)) is a salt derived from a strong acid (nitric acid) and a weak base (ammonia). It forms an acidic solution and cannot have a pH of 9.

Step 5: Evaluating \\( CH_3COONH_4 \\\)

Ammonium acetate (\( CH_3COONH_4 \)) is a salt from a weak acid and weak base. It typically results in a neutral solution, not explaining a pH of 9.

Step 6: Conclusion: Identifying the Correct Salt

Based on the analysis of each salt, sodium acetate (\( CH_3COONa \)) is the only salt that dissolves in water to produce a basic solution with a pH around 9.

Key Concepts

Acid-Base Reactions

When we talk about acid-base reactions, we're diving into how acids and bases interact with each other. These reactions often result in the formation of a salt and water. Here's an interesting point: in such reactions, the strength of the acid and the base plays a crucial role in determining the pH of the resulting solution.
For instance, if the acid is weak and the base is strong, like in the case of acetic acid and sodium hydroxide which form sodium acetate, the resulting solution tends to be basic. Conversely, if a strong acid reacts with a weak base, the solution can be acidic. Here are a few key ideas to remember about these reactions:

• Acids release hydrogen ions ( H^+ ) into solutions.
• Bases release hydroxide ions ( OH^− ).
• Sodium acetate is formed from a weak acid and a strong base, making its solution basic.

Comprehending these relationships helps in predicting whether a salt will produce an acidic, basic, or neutral solution.

Salt Solubility

The solubility of a salt in water is an essential factor in understanding the resulting solution's properties. Soluble salts dissociate into their ions in water, which can affect the pH of the solution. In the example exercise, calcium carbonate ( CaCO_3 ) is mentioned. Despite being a salt, it is not readily soluble in water. Thus, it wouldn't contribute significantly to a solution's pH.
Sodium acetate ( CH_3COONa ), however, dissolves easily, which is why it significantly impacts the solution's pH at around 9. Here is what you need to know about salt solubility:

• Soluble salts dissociate completely in water, contributing to the pH.
• Some insoluble salts do not affect the pH much due to limited dissociation.
• A salt's solubility product can influence its behavior in solutions.

Understanding these details helps in predicting the solubility and subsequent effect of various salts on solution pH.

Solution pH

The pH of a solution is a measure of its acidity or alkalinity (basic nature). It's determined by the concentration of hydrogen ions ( H^+ ), where a higher concentration results in lower pH, indicating acidity. For basicity, the more hydroxide ions ( OH^− ) present, the higher the pH.
In a pH scale ranging from 0 to 14, a pH of 7 is neutral, below 7 is acidic, and above 7 is basic. The exercise example mentions a pH of 9 for sodium acetate solution, highlighting its basic nature. Here's how you can think about solution pH:

• A pH of below 7 means hydrogen ions dominate.
• A pH above 7 indicates abundance of hydroxide ions.
• The pH can change upon adding certain solutions, applicable in buffer solutions or through dilution.

Accurately assessing a solution's pH can guide us in understanding the resulting reactions and the environment in which different chemical processes occur.