For buffer we use Hendersen-Hasselbalck equation,
pH = pKa + log([base]/acid])
4.2 = 4.76 + log([acetate]/acetic acid])
[acetate] = 0.275[acetic acid]
we know,
[acetate] + [acetic acid] = 0.1 M x 0.8 L = 0.08 mols
0.275[acetic acid] + [acetic acid] = 0.08
[acetic acid] = 0.024 mols
[acetate] = 0.056 mols
Volume of 0.1 M CH3COOH needed for buffer preparation = 0.024/0.1 = 0.24 L
Volume of 0.1 M CH3COONa needed for buffer preparation = 0.056/0.1 = 0.56 L
So, we would mix 240 ml of 0.1 M CH3COOH and 560 ml of 0.1 M CH3COONa
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