When a small amount of HCI is added to a buffer solution of acetic acid and sodium acetate

Options

A. pH increases
B. `[H^+]` decreases
C. Dissociation of acetic acid decreases
D. `[CH_3COO^-]` increases

Answer

To solve the problem of what happens when a small amount of HCl is added to a buffer solution of acetic acid and sodium acetate, we can follow these steps:

### Step 1: Understand the Components of the Buffer Solution
The buffer solution consists of acetic acid (CH₃COOH), which is a weak acid, and sodium acetate (CH₃COONa), which provides the conjugate base (CH₃COO⁻) of the weak acid.

**Hint:** Remember that a buffer solution resists changes in pH when small amounts of acids or bases are added.

### Step 2: Identify the Effect of Adding HCl
When HCl, a strong acid, is added to the buffer solution, it dissociates completely in water to produce H⁺ ions. This increases the concentration of H⁺ ions in the solution.

**Hint:** Strong acids dissociate completely, leading to an increase in H⁺ concentration.

### Step 3: Analyze the Buffer Action
The buffer system works to resist changes in pH. The added H⁺ ions will shift the equilibrium of the dissociation of acetic acid. The equilibrium reaction can be represented as:
\[ \text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+ \]

With the increase in H⁺ ions, the equilibrium will shift to the left to reduce the concentration of H⁺ ions by converting some of the CH₃COO⁻ back to CH₃COOH.

**Hint:** The Le Chatelier's principle states that if a system at equilibrium is disturbed, the system will adjust to counteract the disturbance.

### Step 4: Determine the Changes in Concentration
As the equilibrium shifts to the left, the concentration of acetic acid (CH₃COOH) will increase, while the concentration of acetate ions (CH₃COO⁻) will decrease. This means that the dissociation of acetic acid decreases.

**Hint:** Look for changes in the concentrations of the components in the buffer solution.

### Step 5: Evaluate the Options
Now, let's evaluate the options given in the question:
1. **pH increases** - Incorrect, because the buffer resists changes in pH.
2. **Concentration of H⁺ decreases** - Incorrect, the concentration of H⁺ actually increases initially.
3. **Dissociation of acetic acid decreases** - Correct, as the equilibrium shifts to the left.
4. **CH₃COO⁻ increases** - Incorrect, the concentration of acetate ions decreases.

**Hint:** Carefully analyze each option based on the changes in equilibrium and concentration.

### Conclusion
The correct answer is that the dissociation of acetic acid decreases when HCl is added to the buffer solution of acetic acid and sodium acetate.