Answer
(a) $0.1 \space M$ $NaCl$ has the higher pH.
(b) $0.1 \space M$ $Co(NO_3)_2$ has the higher pH.
Work Step by Step
(a) $Ni^{+}$ acts as an acid, and $Cl^-$ is neutral, which results in an acidic solution.
Both $Na^+$ and $Cl^-$ are neutral, which results in a neutral solutions.
Acidic solutions $(pH \lt 7)$ always have lower pH than a neutral solution (pH = 7).
(b) $N{O_3}^-$ is neutral, and $Sn^{2+}$ acts as an acid: Acidic solution.
$N{O_3}^-$ is neutral, and $Co^{2+}$ acts as an acid: Acidic solution.
Since both are acidic, we have to compare the Ka of the acids.
With a lower $K_a$, we have less $H_3O^+$ at equilibrium, which means a higher pH;
According to Appendix C, $Co^{2+}$ has the lower $K_a$, therefore, it forms the solution with higher pH.
