The first and second ionization constants of $\mathrm{H}_2 \mathrm{X}$ are $2.5 \times 10^{-8}$ and $1.0 \times 10^{-13}$ respectively. The concentration of $\mathrm{X}^{2-}$ in $0.1 \mathrm{M} \mathrm{H}_2 \mathrm{X}$ solution is
$\_\_\_\_$ $\times 10^{-15} \mathrm{M}$. (Nearest Integer)
