The pH of a 0.01 N monobasic acid is 4. The acid must be

Options

A. strong
B. weak
C. same
D. different

Answer

To determine the type of acid based on the given pH and normality, we can follow these steps:

### Step 1: Understand the given information
We have a monobasic acid with a normality of 0.01 N and a pH of 4.

### Step 2: Calculate the concentration of hydrogen ions \([H^+]\)
The pH is given by the formula:
\[ \text{pH} = -\log[H^+] \]
From the pH of 4, we can find the concentration of hydrogen ions:
\[ [H^+] = 10^{-\text{pH}} = 10^{-4} \, \text{M} \]

### Step 3: Relate the concentration of hydrogen ions to the acid's normality
Since the acid is monobasic, the normality (N) is equal to the molarity (M) for monobasic acids. Therefore, the concentration of the acid is also 0.01 M.

### Step 4: Compare the expected \([H^+]\) concentration with the calculated one
For a strong acid, we would expect the concentration of hydrogen ions to be equal to the concentration of the acid. Thus, for a 0.01 M strong acid, we would expect:
\[ [H^+] = 0.01 \, \text{M} = 10^{-2} \, \text{M} \]
However, we calculated \([H^+] = 10^{-4} \, \text{M}\). This indicates that the acid does not fully dissociate.

### Step 5: Conclude the type of acid
Since the actual \([H^+]\) concentration (10^{-4} M) is much lower than the expected concentration for a strong acid (10^{-2} M), we can conclude that the acid is a weak acid.

### Final Conclusion
The monobasic acid must be a weak acid.

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