A buffer of pH 5.0 is needed. Four weak acids have $p K_{a}$ values: HA1 3.2, HA2 4.1, HA3 5.4, HA4 7.0. Using the rule that a buffer works well when $∣ pH - p K_{a} ∣ \leq 1$ , which of them (with its sodium salt) are suitable?

Answer: B, C

Differences from pH 5.0 are 1.8, 0.9, 0.4 and 2.0. Only HA2 and HA3 lie within 1 unit, so they are suitable.

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