Select the incorrect statement:

Options

A. pH of 500 ml 0.001 N `HNO_(3)` is 3.0
B. pH of 500 ml 0.001 N `HNO_(3)` is 4 - log 5
C. pH of 20 ml decinormal HCI solution is 1.0
D. pH of 100 ml 0.01 N NaCI solution is = 7.0

Answer

To solve the question of selecting the incorrect statement regarding the pH of various solutions, we will analyze each statement step by step.

### Step 1: Analyze the first statement
**Statement:** The pH of 500 ml of 0.001 normal HNO3 is 3.

**Solution:**
1. **Determine the concentration of H⁺ ions:** HNO3 is a strong acid and dissociates completely. Therefore, the concentration of H⁺ ions is the same as the concentration of HNO3.
2. **Given:** Normality (N) = 0.001 N, which is also the same as molarity (M) since HNO3 has an n-factor of 1.
3. **Calculate pH:**
- pH = -log[H⁺]
- pH = -log(0.001) = -log(10^-3) = 3.
4. **Conclusion:** This statement is correct.

### Step 2: Analyze the second statement
**Statement:** The pH of 500 ml of 0.001 HNO3 is 4 - log 5.

**Solution:**
1. **From Step 1:** We already calculated the pH of 0.001 HNO3 to be 3.
2. **Evaluate the statement:**
- 4 - log 5 does not equal 3.
- log 5 is approximately 0.699, so 4 - log 5 ≈ 4 - 0.699 ≈ 3.301, which is not equal to 3.
3. **Conclusion:** This statement is incorrect.

### Step 3: Analyze the third statement
**Statement:** The pH of 20 ml of 0.1 normal HCl is 1.

**Solution:**
1. **Determine the concentration of H⁺ ions:** HCl is also a strong acid and dissociates completely.
2. **Given:** Normality (N) = 0.1 N, which is the same as molarity (M) since HCl has an n-factor of 1.
3. **Calculate pH:**
- pH = -log[H⁺]
- pH = -log(0.1) = -log(10^-1) = 1.
4. **Conclusion:** This statement is correct.

### Step 4: Analyze the fourth statement
**Statement:** The pH of 100 ml of 0.1 NaCl solution is 7.

**Solution:**
1. **Understanding NaCl:** NaCl is a neutral salt formed from a strong acid (HCl) and a strong base (NaOH).
2. **pH of neutral salts:** The pH of a solution of NaCl is neutral (pH = 7) because it does not affect the concentration of H⁺ or OH⁻ ions in water.
3. **Conclusion:** This statement is correct.

### Final Conclusion
After analyzing all statements:
- The first statement is correct.
- The second statement is incorrect.
- The third statement is correct.
- The fourth statement is correct.

Thus, the **incorrect statement** is the second one: "The pH of 500 ml of 0.001 HNO3 is 4 - log 5."