What volume of \(0.2M \, CH_3COOH\) needs to be added to \(100 \, ml\) of \(0.4M \, CH_3COONa\) solution to prepare a buffer of pH equal to \(4.91\)? Given that \(pK_a\) of \(CH_3COOH\) is \(4.76\).
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For acidic buffer questions, use \(pH = pK_a + \log \frac{[\text{Salt}]}{[\text{Acid}]}\). Always calculate moles first, then convert them into volume using molarity.