A : `H_3O^+` ion from water is also taken in consideration while calculating the pH of very dilute solution (say concentration = `10^(-9)` M ) of acid
R: `[H_3O^+]` from water is only available in very dilute solution of acid.

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 assertion-reasoning question, we need to analyze both the assertion (A) and the reasoning (R) provided.

### Assertion (A):
**"H₃O⁺ ion from water is also taken into consideration while calculating the pH of very dilute solution (say concentration = 10^(-9) M) of acid."**

### Reasoning (R):
**"[H₃O⁺] from water is only available in very dilute solution of acid."**

### Step-by-Step Solution:

1. **Understanding pH Calculation**:
- The pH of a solution is calculated using the formula:
\[ \text{pH} = -\log[H^+] \]
- In very dilute solutions, the concentration of hydrogen ions (H⁺) from the acid may be comparable to the concentration of H⁺ ions from the self-ionization of water.

2. **Self-Ionization of Water**:
- Water undergoes self-ionization:
\[ 2H_2O \rightleftharpoons H_3O^+ + OH^- \]
- The ion product of water (K_w) at 25°C is:
\[ K_w = [H_3O^+][OH^-] = 10^{-14} \]
- In pure water, the concentration of H₃O⁺ is:
\[ [H_3O^+] = [OH^-] = 10^{-7} \, \text{M} \]

3. **Analyzing the Given Concentration**:
- For a very dilute acid solution (e.g., 10^(-9) M), the contribution of H₃O⁺ from the acid is:
\[ [H_3O^+] = 10^{-9} \, \text{M} \]
- However, the contribution from water (self-ionization) is:
\[ [H_3O^+] = 10^{-7} \, \text{M} \]

4. **Total H₃O⁺ Concentration**:
- When calculating the total concentration of H₃O⁺ in the solution, we must consider both contributions:
\[ [H_3O^+]_{\text{total}} = [H_3O^+]_{\text{acid}} + [H_3O^+]_{\text{water}} = 10^{-9} + 10^{-7} \]
- Since \(10^{-7} \, \text{M}\) is significantly larger than \(10^{-9} \, \text{M}\), we cannot neglect the contribution from water.

5. **Calculating pH**:
- Thus, the effective concentration of H₃O⁺ in the solution is approximately \(10^{-7} \, \text{M}\).
- Therefore, the pH would be:
\[ \text{pH} = -\log(10^{-7}) = 7 \]

6. **Conclusion on Assertion**:
- The assertion is **true** because the H₃O⁺ from water must be considered in very dilute solutions.

7. **Evaluating the Reasoning**:
- The reasoning states that H₃O⁺ from water is only available in very dilute solutions. This is **false** because H₃O⁺ from water is always present, regardless of the solution's dilution; it just becomes negligible in more concentrated solutions.

### Final Answer:
- Assertion (A) is **true**.
- Reasoning (R) is **false**.
- Therefore, the correct option is: **Assertion is true, but Reason is false.**