Methyl orange became red=very acidic solution. The concentration of A

^{2-} is very low, so it means that K

_{a1} is enough big (therefore the solution is acidic).

H

_{2}A

HA

^{-}+H

^{+} K

_{a1} reasonable big

HA

^{-} A

^{2-}+H

^{+} K

_{a2}=?

If I mark the concentration of H

_{2}A by x, then the concentration of HA

^{-} and H

^{+} are both x, too.

K

_{a2}=[A

^{2-}]*[H

^{+}]/[HA

^{-}]

The concentration of A

^{2-} is very low, meaning that K

_{a2} is very low, too, so the amount of H

^{+} produced can be neglected to the amount of H

^{+} produced in the first step. The amount of HA

^{-} that dissociated can also be neglected, so:

K

_{a2}=5.1×10

^{-10}*x/x=5.1×10

^{-10}pK

_{a2}≈9.3.