The first and second dissociation constants of an acid ${\mathrm{H}}_2\mathrm{A}$ and $1.0\times {10}^{-5}$ and $5.0\times {10}^{-10}$ respectively. The overall dissociation constant of the acid will be:

$k_{\text{overall }}=k_1\times k_2$

The overall dissociation constant is equal to the product of first and second dissociation constant. It is ${\mathrm{K}}_{\mathrm{a}}={\mathrm{K}}_{\mathrm{a}1}\times {\mathrm{K}}_{\mathrm{a}2}=1.0\times {10}^{-5}\times 5.0\times {10}^{-10}=5.0\times {10}^{-15}$