Calculate the pH of the mixture of $ 2 \ L \; CH_3COOH \; 0.1M $ with $ 1 \ L \; CH_3COONa \; 0.2M $ Explain why this mixture is a buffer!
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$CH_3COONa$ is a water-soluble salt which dissociates therefrom to form the weak base $ CH_3COO^- $ and $ Na^+ $ which has no influence on pH. We are thus in the presence of a mixture of a weak acid $ CH_3COOH $ with its corresponding base $ CH_3COO^- $, hence of a buffer!
Calculate the number of moles of the two species!
$n_{CH_3COOH}$ $=$ $2\cdot 0.1$ $=$ $0.2\; mol$ $n_{CH_3COO^-}$ $=$ $1\cdot 0.2$ $=$ $0.2 \;mol$
Calculate the pH !
$pH$ $=$ $4.75+log\frac{0.2}{0.2}$ $=$ $4.75$
