Question

A weak monobasic acid is 0.04 % dissociated in 0.25 M solution. What is pH of the solution?

• A. \(2.5\)
• B. \(4\)
• C. \(5\)
• D. \(10\)

Hint:

Convert the percentage to a degree of dissociation, multiply by the concentration to get the hydrogen ion concentration, then take the negative log.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
For a weak monobasic acid HA, each molecule that dissociates gives one \(\text{H}^+\). So \([\text{H}^+] = C\alpha\), where \(C\) is the initial concentration and \(\alpha\) is the degree of dissociation.

Step 2: Key Formula or Approach:
\[ [\text{H}^+] = C\alpha, \qquad \text{pH} = -\log[\text{H}^+] \]

Step 3: Detailed Explanation:
Percentage dissociation is 0.04 %, so \(\alpha = \dfrac{0.04}{100} = 4 \times 10^{-4}\).
\[ [\text{H}^+] = 0.25 \times 4 \times 10^{-4} = 1 \times 10^{-4}\text{ M} \]
\[ \text{pH} = -\log(10^{-4}) = 4 \]

Step 4: Why the other options are wrong.
pH 2.5 and pH 5 would need \([\text{H}^+]\) of about \(3.2\times10^{-3}\) M and \(10^{-5}\) M. pH 10 is basic, which cannot be true for an acid solution. Only pH 4 matches \(10^{-4}\) M.

Final Answer:
The hydrogen ion concentration is \(10^{-4}\) M, so the pH is 4, option (B). \[ \boxed{4} \]