When a salt of weak acid and weak base is dissolved in water, the pH of the resulting solution will be :
(a)be 7
(b)be greater than 7
(c)be less than 7
(d)depend upon `K_(a)` and `K_(b)` values
A
be 7
B
be greater than 7
C
be less than 7
D
depend upon `K_(a)` and `K_(b)` values
Text Solution
AI Generated Solution
The correct Answer is:
To determine the pH of a solution when a salt of a weak acid and a weak base is dissolved in water, we can analyze the situation step by step.
### Step-by-Step Solution:
1. **Understanding the Components**:
- A salt formed from a weak acid and a weak base will dissociate in water to produce its ions.
- The weak acid (HA) partially dissociates to give H⁺ ions, while the weak base (B) partially dissociates to give OH⁻ ions.
2. **Dissociation of the Salt**:
- When the salt (e.g., AB) is dissolved in water, it dissociates into A⁻ (the conjugate base of the weak acid) and B⁺ (the conjugate acid of the weak base).
- The reaction can be represented as:
\[
AB \rightleftharpoons A^- + B^+
\]
3. **Buffer Formation**:
- The presence of both A⁻ and B⁺ creates a buffer solution. A buffer solution can resist changes in pH upon the addition of small amounts of acids or bases.
- The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation:
\[
\text{pH} = \text{pK}_a + \text{pK}_b
\]
- Where \( \text{pK}_a \) is the negative logarithm of the acid dissociation constant (Kₐ) of the weak acid, and \( \text{pK}_b \) is the negative logarithm of the base dissociation constant (Kᵦ) of the weak base.
4. **Determining the pH**:
- Since both Kₐ and Kᵦ are involved, the resulting pH will depend on the relative strengths of the weak acid and weak base.
- If Kₐ is significantly greater than Kᵦ, the solution will be more acidic (pH < 7). Conversely, if Kᵦ is significantly greater than Kₐ, the solution will be more basic (pH > 7).
- Therefore, the pH cannot be determined without knowing the specific values of Kₐ and Kᵦ.
5. **Conclusion**:
- The pH of the solution formed by dissolving a salt of a weak acid and a weak base depends on the values of Kₐ and Kᵦ. Thus, the correct answer is option (d) "depend upon Kₐ and Kᵦ values".
