Ionic Equilibria In Solution

Step 1: For a basic buffer (weak base BOH and its conjugate acid salt BA), use the Henderson-Hasselbalch equation for bases: Given: , , .
Step 2: Calculate p :
Step 3: Calculate pOH:
Step 4: Calculate pH:

For acetic acid (weak acid), ,
Initial concentration , .
Let , .
Answer: 1.34 × 10 M.

For a weak monobasic acid , the dissociation constant , where is the degree of dissociation, and is concentration.
Step 1: For 0.05 M, :
Step 2: For 0.15 M, use : Since is small, approximate : Percent dissociation = .

Part 1: Definition of Conjugate Acid-Base Pair A conjugate acid-base pair consists of two species that differ from each other by the presence of a single proton (H ). When a Brønsted-Lowry acid donates a proton, the species that remains is its conjugate base. When a Brønsted-Lowry base accepts a proton, the species formed is its conjugate acid.
Example: In the reaction , HCl is the acid and Cl is its conjugate base. H O is the base and H O is its conjugate acid.

Part 2: Calculation of pH
Step 1: Given Information
Hydroxyl ion concentration, [OH ] = 2 10 mol dm .
Step 2: Calculate pOH
The pOH is the negative logarithm of the hydroxyl ion concentration.
Step 3: Calculate pH The sum of pH and pOH at 298 K (25 C) is 14. The pH of the solution is 10.301.

Step 1: Understanding the Concept:
Acetic acid is a weak electrolyte. Its dissociation constant ( ) is related to its concentration ( ) and degree of dissociation ( ) by Ostwald's dilution law.
Step 2: Key Formula or Approach:
For a weak acid:

If is very small ( ), the formula can be simplified to:

Step 3: Detailed Explanation:
Given:
Degree of ionisation, .
Concentration, .
Using the formula:




Step 4: Final Answer:
The dissociation constant ( ) of acetic acid is .

Step 1: Understanding the Concept:
This effect is based on Le Chatelier's principle applied to ionic equilibria.
Step 2: Detailed Explanation:
When a strong electrolyte that provides an ion already present in the solution of a weak electrolyte is added, the equilibrium of the weak electrolyte shifts in the direction of the undissociated molecule.
Example: Adding (strong) to (weak). The increased concentration of ions drives the dissociation of acetic acid backwards.

Step 3: Final Answer:
Common ion effect is the decrease in the degree of dissociation of a weak electrolyte upon adding a strong electrolyte with a common ion.

Step 1: Understanding the Concept:
Amphoteric substances are those that can react as both acids (proton donors) and bases (proton acceptors) depending on the reacting species.
Step 2: Detailed Explanation:
1. Water as a base: When water reacts with a stronger acid like , it accepts a proton.

2. Water as an acid: When water reacts with a base like , it donates a proton.

Since water behaves as an acid in the presence of bases and as a base in the presence of acids, it is described as amphoteric or amphiprotic.
Step 3: Final Answer:
The ability of water to donate or accept protons makes it amphoteric.

Step 1: Understanding the Concept:
HCl is a strong acid that dissociates completely in water.

The concentration of ions is equal to the concentration of the acid for a monoprotic strong acid.
Step 2: Key Formula or Approach:
1.
2. (at 298 K)
Step 3: Detailed Explanation:
Given:
Therefore, .
Calculate pH:

Calculate pOH:


Step 4: Final Answer:
The pOH of the solution is 12.