Answer
$0.10M$ $H_2CO_3$ is more acidic.
Work Step by Step
** You can find the $K_a$ values on table: 14.2 (Page 624)
Since both solutions have the same concentration, we just have to determine which compound is more acidic:
-$H_2CO_3$:
$H_2CO_3$ is an acid with $K_a = 4.3 \times 10^{-7}$
- $NH_4Cl$:
It is a salt formed by $N{H_4}^+$ and $Cl^-$.
$Cl^-$ has negligible basicity, so, we don't need to consider it.
$N{H_4}^+$ is an acid with $K_a = 5.6 \times 10^{-10}$
$H_2CO_3$ has a greater $K_a$ value, therefore, it is more acidic.
So $0.10M$ $H_2CO_3$ is more acidic.
