Why does adding a common ion suppress the degree of dissociation $\alpha$ of a weak electrolyte?

According to Le Chatelier's principle, adding an ion already present in the equilibrium mixture increases the concentration of product ions. To relieve this stress, the system shifts backwards, forcing free ions to recombine into undissociated molecules, thereby reducing $\alpha$.

How do I decide whether to approximate $1 - \alpha \approx 1$ in equilibrium calculations?

As a Kota standard rule, if $\alpha \le 0.05$ (or $5\%$) or if $\frac{C}{K_a} \ge 400$, you can safely approximate $1 - \alpha \approx 1$. If $\alpha > 0.05$, you MUST solve the full quadratic equation $K_a = \frac{C\alpha^2}{1 - \alpha}$.

What is buffer capacity and when is an acidic buffer solution most effective?

Buffer capacity quantifies a solution's resistance to $\text{pH}$ change upon addition of strong acid or base. An acidic buffer exhibits maximum buffer capacity when $[\text{Salt}] = [\text{Acid}]$, which corresponds to $\text{pH} = \text{p}K_a$.

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Question #1NEET 2023

Find the $\text{pH}$ of a $0.01\text{ M}$ solution of ammonium acetate $(\text{CH}_3\text{COONH}_4)$ at $298\text{ K}$ . Given $\text{p}K_a(\text{CH}_3\text{COOH}) = 4.76$ and $\text{p}K_b(\text{NH}_4\text{OH}) = 4.75$ .

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Step-by-Step Solution & Concept

Step 1

Identify that ammonium acetate $(\text{CH}_3\text{COONH}_4)$ is a salt of a weak acid $(\text{CH}_3\text{COOH})$ and a weak base $(\text{NH}_4\text{OH})$ .

Step 2

Apply the salt hydrolysis formula for weak acid - weak base salt: $\text{pH} = 7 + \frac{1}{2}(\text{p}K_a - \text{p}K_b)$ .

Step 3

Substitute the given values: $\text{pH} = 7 + \frac{1}{2}(4.76 - 4.75) = 7 + \frac{1}{2}(0.01) = 7.005$ .

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Question #2JEE Main 2024

For the gaseous reaction $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ , the value of $K_p$ is $4.9 \times 10^{-5}\text{ atm}^{-2}$ at $500^\circ\text{C}$ . Calculate $K_c$ for the reaction at the same temperature. (Take $R = 0.0821\text{ L atm K}^{-1}\text{ mol}^{-1}$ )

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Step-by-Step Solution & Concept

Step 1

Convert temperature to Kelvin: $T = 500 + 273.15 = 773.15\text{ K}$ , and calculate change in gaseous moles: $\Delta n_g = 2 - (1 + 3) = -2$ .

Step 2

Rearrange relation $K_p = K_c (RT)^{\Delta n_g}$ to get $K_c = K_p (RT)^{-\Delta n_g} = K_p (RT)^2$ .

Step 3

Substitute values: $K_c = 4.9 \times 10^{-5} \times (0.0821 \times 773.15)^2 = 4.9 \times 10^{-5} \times (63.476)^2 \approx 1.97 \times 10^{-1}\text{ M}^{-2}$ .

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Mastering Equilibrium (Chemical & Ionic): Formulas, Traps & Exam Strategy | ExamiFly

Chapter Overview

Core Subtopics & Syllabus Breakdown

Key Formulas & High-Yield Equations

Relation Between $K_p$ and $K_c$

Ostwald's Dilution Law

Henderson-Hasselbalch Equation (Acidic Buffer)

pH of Weak Acid and Weak Base Salt Hydrolysis

Condition for Precipitation

Common Mistakes & Traps to Avoid

Past Year Question (PYQ) Trends

Solved High-Yield Sample Questions

Frequently Asked Questions

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