Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

The \(\mathrm{pH}\) values of \(1 \mathrm{M}\) solutions of \(\mathrm{CH}_{3} \mathrm{COOH}\) (I), \(\mathrm{CH}_{3}, \mathrm{COONa}(\mathrm{II}), \mathrm{CH}_{3} \mathrm{COONH}_{4}(\mathrm{III})\), and \(\mathrm{KOH}\) (IV) will be in the order (a) IV \(>I I I>I I>I\) (b) IV \(>\mathrm{II}>\mathrm{III}>\mathrm{I}\) (c) \(\mathrm{I}>\mathrm{III}>\mathrm{II}>\mathrm{IV}\) (d) \(\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}\)

Step-by-Step Solution

Answer

Option (b): IV > II > III > I.

Step 1: Identify Nature of Solutions

First, identify the nature of each compound. \(\begin{align*}\text{I:} & \ \mathrm{CH}_3\mathrm{COOH} \text{ is a weak acid.} \\text{II:} & \ \mathrm{CH}_3\mathrm{COONa} \text{ is a salt derived from a weak acid and strong base, considered basic.} \\text{III:} & \ \mathrm{CH}_3\mathrm{COONH}_4 \text{ is a salt derived from a weak acid and weak base, and is generally neutral or slightly acidic.} \\text{IV:} & \ \mathrm{KOH} \text{ is a strong base.}\end{align*}\)

Step 2: Rank Based on Strength

Now, compare the compounds based on their acidic or basic nature. Strong bases have higher \(\mathrm{pH}\) and acidic solutions have lower \(\mathrm{pH}\).- Compound IV (\(\mathrm{KOH}\)) is a strong base, so it will have the highest \(\mathrm{pH}\).- Compound II (\(\mathrm{CH}_3\mathrm{COONa}\)) is basic, due to the strong base origin, so it has the next highest \(\mathrm{pH}\).- Compound III (\(\mathrm{CH}_3\mathrm{COONH}_4\)) is neutral or slightly acidic, thus it falls next.- Compound I (\(\mathrm{CH}_3\mathrm{COOH}\)) is an acid, thus has the lowest \(\mathrm{pH}\).

Step 3: Determine Correct Order

From the analysis in Step 2, we determine:- The order of \(\mathrm{pH}\) is IV > II > III > I- This matches option (b) from the given options: IV > II > III > I.

Key Concepts

Acid-Base Solutions

When studying solutions, it's key to identify whether they are acidic or basic. Acids and bases are two types of chemical substances that react in solutions to alter the pH levels. Here are some simple aspects to keep in mind:- **Acids**: They donate protons (\( H^+ \)) when dissolved in water, increasing the number of \( H^+ \) ions. Thus, they typically have a low pH. For example, \( \mathrm{CH}_3\mathrm{COOH} \) (acetic acid) dissociates to produce protons, making it a weak acid.- **Bases**: These are proton acceptors and increase the number of hydroxide ions (\( OH^- \)) in solutions. Compounds like \( \mathrm{KOH} \) are strong bases, as they completely dissociate in water to release \( OH^- \) ions. This results in high pH solutions.- **Salts**: Derived from acids and bases, they can be neutral, acidic, or basic when dissolved in water. For example: - \( \mathrm{CH}_3\mathrm{COONa} \) is formed from a strong base and a weak acid, making it slightly basic. - \( \mathrm{CH}_3\mathrm{COONH}_4 \), on the other hand, comes from weak acid and weak base, often resulting in neutral or slightly acidic solutions.Understanding the nature of these substances helps us predict the behavior of solutions when mixed, which is crucial in experiments and calculations.

pH Ranking

Ranking the pH of various solutions requires understanding their acid-base nature. The measure of acidity or basicity of a solution is expressed in terms of pH.The pH scale ranges from 0 to 14:- **pH 7** is neutral, like pure water.- **pH less than 7** indicates acidic solutions.- **pH greater than 7** shows basic solutions.A strong base like \( \mathrm{KOH} \) will have a pH close to 14 as it fully dissociates, releasing many \( OH^- \) ions. In contrast, a weak acid such as \( \mathrm{CH}_3\mathrm{COOH} \) will have a pH closer to 3-4, given it only partially dissociates. When ranking solutions:- Compounds with stronger acidic or basic nature skew the pH further towards their respective end of the scale. - Therefore, \( \mathrm{KOH} \) > \( \mathrm{CH}_3\mathrm{COONa} \) > \( \mathrm{CH}_3\mathrm{COONH}_4 \) > \( \mathrm{CH}_3\mathrm{COOH} \) is the correct order due to their respective strengths in dissociation and origin.

Compound Analysis

To fully understand how these compounds behave and influence pH, let's break them down:- **Acids and Their Derivatives**: - \( \mathrm{CH}_3\mathrm{COOH} \) is a simple organic acid also called acetic acid. - Its partially dissociating nature means it only releases some of its hydrogen ions, remaining as a weak acid with low pH.- **Bases and Salts**: - \( \mathrm{KOH} \) is a strong base; when in solution, it dissociates completely, raising pH significantly due to abundant \( OH^- \) ions. - \( \mathrm{CH}_3\mathrm{COONa} \) emerges from the neutralization of \( \mathrm{CH}_3\mathrm{COOH} \) with \( \mathrm{NaOH} \), a strong base leading it to form a basic solution. - \( \mathrm{CH}_3\mathrm{COONH}_4 \) forms from \( \mathrm{CH}_3\mathrm{COOH} \) neutralizing \( \mathrm{NH}_4OH \) (ammonium hydroxide). Both being weak, they form an almost neutral solution.Analyzing the source and nature of these compounds not only helps predict their behavior in water but also helps in calculating pH accurately.