Since $H_2A$ is a diprotic acid, it will dissociate in two steps. The first dissociation is:
$H_2A \rightleftharpoons H^+ + HA^-$
$K_{a1} = \frac{[H^+][HA^-]}{[H_2A]} = 2.8 \times 10^{-6}$
Let x be the concentration of $H^+$ and $HA^-$ at equilibrium. Then, the
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