Question

Calculate pH of buffer solution composed of 0.1 M weak base BOH and 0.2 M of its salt BA. (\( K_b = 1.8 \times 10^{-5} \) for weak base).

Hint:

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Answer 1

Step 1: For a basic buffer (weak base BOH and its conjugate acid salt BA), use the Henderson-Hasselbalch equation for bases: \[ \text{pOH} = \text{p}K_b + \log \frac{[\text{salt}]}{[\text{base}]}. \] Given: \( [\text{BOH}] = 0.1 \, \text{M} \), \( [\text{BA}] = 0.2 \, \text{M} \), \( K_b = 1.8 \times 10^{-5} \).
Step 2: Calculate p\( K_b \): \[ \text{p}K_b = -\log (K_b) = -\log (1.8 \times 10^{-5}) \approx 4.74. \]
Step 3: Calculate pOH: \[ \text{pOH} = 4.74 + \log \frac{0.2}{0.1} = 4.74 + \log 2 \approx 4.74 + 0.30 = 5.04. \]
Step 4: Calculate pH: \[ \text{pH} = 14 - \text{pOH} = 14 - 5.04 = 9.26. \]