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Problems · Problem 3.10

Q.Calculate the pH of buffer solution composed of 0.1 M weak base BOH and 0.2 M of its salt BA. [Kb=1.8×10−5K_b = 1.8\times 10^{-5} for the weak base]

Maharashtra MsbshseTextbookSubjectiveImportance★★★★★
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pKb=4.7447K_b = 4.7447 and log⁡(0.2/0.1)=0.3010\log(0.2/0.1) = 0.3010 give pOH =5.0457= 5.0457 and pH =8.9543= 8.9543.

Step 1. BOH (weak base) with its salt BA is a basic buffer, so the Henderson-Hasselbalch equation in its basic form (Eq. 3.26) applies: pOH=pKb+log⁡10[salt][base]\text{pOH} = \text{p}K_b + \log_{10}\dfrac{[\text{salt}]}{[\text{base}]}, with [salt]=0.2[\text{salt}] = 0.2 M and [base]=0.1[\text{base}] = 0.1 M.

Step 2. pKb=−log⁡10(1.8×10−5)=5−log⁡101.8=5−0.2553=4.7447\text{p}K_b = -\log_{10}(1.8\times10^{-5}) = 5 - \log_{10}1.8 = 5 - 0.2553 = 4.7447.

Step 3. log⁡100.20.1=log⁡102=0.3010\log_{10}\dfrac{0.2}{0.1} = \log_{10}2 = 0.3010, so pOH=4.7447+0.3010=5.0457\text{pOH} = 4.7447 + 0.3010 = 5.0457. …

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