A 0.1 M solution of a weak diprotic acid H2A has K_a1 = 1.0x10^-5 and K_a2 = 1.0x10^-10. In this solution (no other acid or base added) the concentration of A^2- is approximately

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A $0.1 M$ solution of a weak diprotic acid $H X_{2} A$ has $K_{a 1} = 1.0 \times 1 0^{- 5}$ and $K_{a 2} = 1.0 \times 1 0^{- 10}$ . In this solution (no other acid or base added) the concentration of $A X^{2 -}$ is approximately

The first ionisation gives

[H X^{+}] \approx [HA X^{-}] = 1 0^{- 5} \times 0.1 = 1 0^{- 3} M

. Then

[A X^{2 -}] = K_{a 2} [HA X^{-}] / [H X^{+}] = K_{a 2} = 1.0 \times 1 0^{- 10} M