A : pH of mixture of 0.1 M HCN and 0.05 M NaOH is less than 7 .
R : HCN is a weak -acid and NaOH is a strong base.

Options

A. If both Assertion & Reason are true and the reason in the corret explanation of the assertion, then mark
B. If both assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark
C. If Assertion is true statement but Reason is false then mark
D. If both Assertion and Reason are false statements , then mark

Answer

To solve the question regarding the pH of a mixture of 0.1 M HCN and 0.05 M NaOH, we can follow these steps:

### Step 1: Identify the nature of the substances
- HCN (Hydrogen Cyanide) is a weak acid, which means it does not completely dissociate in solution.
- NaOH (Sodium Hydroxide) is a strong base, which completely dissociates in solution.

**Hint:** Remember that strong bases completely dissociate in water, while weak acids do not.

### Step 2: Determine the dissociation of HCN
- The dissociation of HCN in water can be represented as:
\[ \text{HCN} \rightleftharpoons \text{H}^+ + \text{CN}^- \]
- Since HCN is a weak acid, it will establish an equilibrium, and not all of it will dissociate.

**Hint:** Consider the equilibrium constant (Ka) for weak acids to understand how much HCN dissociates.

### Step 3: Determine the dissociation of NaOH
- The dissociation of NaOH can be represented as:
\[ \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- \]
- Given that the concentration of NaOH is 0.05 M, the concentration of OH⁻ ions produced will also be 0.05 M.

**Hint:** Strong bases contribute OH⁻ ions directly to the solution.

### Step 4: Calculate the effect of NaOH on HCN
- The OH⁻ ions from NaOH will react with H⁺ ions from HCN:
\[ \text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O} \]
- The initial concentration of H⁺ ions from HCN is less than 0.05 M, as HCN is a weak acid. Therefore, the OH⁻ ions will neutralize some of the H⁺ ions.

**Hint:** Determine how much H⁺ is produced from HCN and compare it with the OH⁻ ions available.

### Step 5: Determine the resulting concentration of H⁺ ions
- After neutralization, if all OH⁻ ions (0.05 M) react with H⁺ ions, the remaining concentration of H⁺ ions will be from the undissociated HCN.
- Since HCN is a weak acid, the remaining H⁺ concentration will still be present, leading to a solution that is acidic.

**Hint:** If there are still H⁺ ions left after neutralization, the solution will be acidic.

### Step 6: Conclusion about pH
- Since there are still H⁺ ions present in the solution after the reaction with OH⁻, the pH of the mixture will indeed be less than 7.

**Hint:** Remember that a pH less than 7 indicates an acidic solution.

### Final Answer
- **Assertion (A)**: The pH of the mixture of 0.1 M HCN and 0.05 M NaOH is less than 7. **True**
- **Reason (R)**: HCN is a weak acid and NaOH is a strong base. **True**, but it does not explain why the pH is less than 7.

Thus, the correct option is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion.