$pH$ of buffers

Tutorial 5

    

Calculating the pH of a buffer Buffer definition: A buffer is a mixture of a weak base with its conjugated acid pH calculation: In the following calculation only the numbers of moles or molarities before dissociation or before interaction between the species present are included! $ pH $ $ = $ $ pK_a \; + \; \frac{n_{base}} {n_{acid}} $ $ pH $ $ = $ $ pK_a \; + \; \frac{c_{base}} {c_{acid}} $ where $ n $ are the numbers of moles $ c $ the molarities taking into account the dilution during mixing

A chemist needs a buffered solution at $ pH= 4.30$ and may choose between the following weak acids and their soluble salts: (a) Monochloroacetic acid $ (pK_a = $ 2.86) (b) Propanoic acid $ (pK_a = 4.87) $ (c) Hypochlorous acid $ (pK_a = 7.55) $ (d) Benzoic acid $ (pK_a = 4.20) $ Calculate the required $ \frac{n_{base}}{n_{acid}} $ ratio for each buffer to produce $pH=4.30$

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