A solution containing 0.10 M HA and 0.050 M NaA has a measured pH of 4.40 (log 2 = 0.301, 10^-4.40 = 3.98 x 10^-5). Assuming ideal behaviour, K_a of HA is closest to

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A solution containing 0.10 M HA and 0.050 M NaA has a measured pH of 4.40 ( $lo g 2 = 0.301$ , $1 0^{- 4.40} = 3.98 \times 1 0^{- 5}$ ). Assuming ideal behaviour, $K_{a}$ of HA is closest to

K_{a} = [H X^{+}] [A^{-}] / [HA] = 3.98 \times 1 0^{- 5} \times 0.050/0.10 = 1.99 \times 1 0^{- 5}