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 question, we need to analyze both the assertion (A) and the reason (R) provided in the statement.

### Step 1: Understand the Assertion (A)
The assertion states that the `H3O^+` ion from water is also taken into consideration while calculating the pH of a very dilute solution (like `10^(-9)` M) of acid.

**Hint:** Think about how pH is calculated and the role of water in contributing to `H3O^+` concentration.

### Step 2: Understand the Reason (R)
The reason states that `[H3O^+]` from water is only available in very dilute solutions of acid.

**Hint:** Consider whether the contribution of water to `H3O^+` concentration is limited to only very dilute solutions or if it applies to all solutions.

### Step 3: Analyze the Concentration of `H3O^+`
In a neutral solution, the concentration of `H3O^+` from water is `10^(-7)` M at 25°C. When you have a very dilute acid solution (like `10^(-9)` M), the concentration of `H3O^+` from the acid is less than that from water. Therefore, the total concentration of `H3O^+` ions must include both contributions.

**Hint:** Calculate the total `H3O^+` concentration when the acid concentration is very low.

### Step 4: Calculate Total `H3O^+` Concentration
In a `10^(-9)` M acid solution:
- From the acid: `10^(-9)` M
- From water: `10^(-7)` M

Total `[H3O^+]` = `10^(-9) + 10^(-7) = 1.01 × 10^(-7)` M

**Hint:** Remember to add the contributions from both sources to find the total `H3O^+` concentration.

### Step 5: Calculate the pH
Using the total concentration of `H3O^+`, we can calculate the pH:
- pH = `-log(1.01 × 10^(-7))`
- pH ≈ 6.99

**Hint:** Use the logarithmic formula for pH calculation.

### Step 6: Evaluate the Truth of Assertion and Reason
- The assertion (A) is **true** because we do consider `H3O^+` from water in very dilute solutions.
- The reason (R) is **false** because `H3O^+` from water is present in all solutions, not just very dilute ones.

**Hint:** Determine the truth values of both statements to conclude.

### Conclusion
The assertion is true, and the reason is false. Therefore, the correct answer is C.