Question

If a weak acid \(HA\) is \(0.1\%\) ionized (dissociated) in a \(0.2M\) solution, its pH will be _ _ _. (round off to one decimal place)

Hint:

For weak acids: \[ [H^+]=C\alpha \] where \(C\) is concentration and \(\alpha\) is degree of ionization in fractional form.

Answer 1

Correct Answer: 3.7

Step 1: List the data.
Weak acid HA at $0.2\,M$, ionized just $0.1\%$.

Step 2: Get the fraction ionized.
\[ \alpha = \frac{0.1}{100} = 0.001 \]

Step 3: Find the proton concentration.
\[ [H^+] = C\alpha = 0.2 \times 0.001 = 2\times 10^{-4}\ M \]

Step 4: Take the pH.
\[ \text{pH} = -\log(2\times10^{-4}) = 4 - 0.301 = 3.699 \approx 3.7 \]

Step 5: Answer.
\[ \boxed{3.7} \]