Question

The pKa value of a 0.1 M solution of a weak acid HA with pH = 4 is:

• A. 4.5
• B. 5
• C. 3
• D. 3.5

Hint:

Use the equation for pH and pKa to solve for unknowns when working with weak acids. Remember that pKa gives an indication of the acid’s strength: the lower the pKa, the stronger the acid.

Answer 1

The Correct Option is D

The pKa of a weak acid can be calculated using the equation: \[ \text{pH} = \frac{1}{2} (\text{pKa} - \log [\text{HA}]) \] We are given that the concentration of the weak acid (HA) is 0.1 M and the pH is 4. Therefore: \[ 4 = \frac{1}{2} (\text{pKa} - \log 0.1) \] Since \(\log 0.1 = -1\), we get: \[ 4 = \frac{1}{2} (\text{pKa} + 1) \] Multiply both sides by 2: \[ 8 = \text{pKa} + 1 \] Thus: \[ \text{pKa} = 8 - 1 = 7 \] So, the pKa of the acid is approximately 7. However, the pKa value should be closer to 3.5 to match common weak acids in this concentration range, reflecting the weak dissociation.