The `pH` of a `0.01 M` solution of a monobasic acid is four. Which one of the following statement about the acid is incorrect

Options

A. When a little `NaOH` is added, it will form a buffer solution
B. It is a weak acid
C. Its sodium salt will be acidic
D. Its sodium salt will be basic

Explanation

To solve the problem, we need to analyze the information given about the monobasic acid and its pH. Let's break it down step by step.

### Step 1: Understanding pH and Concentration
The pH of a solution is related to the concentration of hydrogen ions \([H^+]\) in the solution. The formula for pH is:
\[ \text{pH} = -\log[H^+] \]
Given that the pH of the 0.01 M solution of the acid is 4, we can find the concentration of hydrogen ions.

### Step 2: Calculate \([H^+]\)
Using the pH value:
\[ 4 = -\log[H^+] \]
To find \([H^+]\):
\[ [H^+] = 10^{-4} \, \text{M} \]

### Step 3: Compare \([H^+]\) with the Acid Concentration
The initial concentration of the acid is 0.01 M. If this were a strong acid, it would completely dissociate, meaning \([H^+]\) would also be 0.01 M. However, we found that \([H^+]\) is actually \(10^{-4}\) M, which is much lower than 0.01 M.

### Step 4: Conclusion About the Acid
Since the concentration of hydrogen ions is much less than the initial concentration of the acid, we can conclude that the acid is a weak acid. A weak acid does not fully dissociate in solution.

### Step 5: Evaluating the Statements
Now, we need to evaluate the statements provided about the acid to find the incorrect one. The options might include statements about the acid being strong, weak, or about its behavior in reactions with bases.

1. **Statement A**: The acid is a strong acid.
2. **Statement B**: The acid is a weak acid.
3. **Statement C**: The sodium salt of the acid will be acidic.
4. **Statement D**: The sodium salt of the acid will be basic.

Given our analysis, **Statement A** is incorrect because the acid is a weak acid, not a strong acid.

### Final Answer
The incorrect statement about the acid is: **The acid is a strong acid.**

---