Answer
$$K_c = 9.5 \times 10^{-27}$$
Work Step by Step
1. Adding both given chemical equations we get:
$$H_2S + HS^- \leftrightharpoons 2 H^+ + HS^- + S^{2-}$$
- Removing the repeated compounds:
$$H_2S \leftrightharpoons 2 H^ + S^{2-}$$
We get exactly the reaction we want.
Since we added the equations, we multiply the $K_c's$ of the original ones:
$$K_c = K'_c \times K''_c = (9.5 \times 10^{-8}) \times (1.0 \times 10^{-19}) = 9.5 \times 10^{-27}$$
