Why is `N_(2)O_(5)` more acidic than `N_(2)O_(3)` ?

Explanation

Since the oxidation state of N is +5 in `N_(2)O_(5)` and +3 in `N_(2)O_(3)`, therefore, `N_(2)O_(5)` can accept electrons more readily than `N_(2)O_(3)` and hence `N_(2)O_(5)` is a stronger acid than `N_(2)O_(3)`/
Alternatively, `N_(2)O_(5)` and `N_(2)O_(3)` dissolve in `H_(2)O` forming `HNO_(3) and HNO_(2)` respectively.

Due to higher oxidation state of +5 of N in `HNO_(3)` than +3 in `HNO_(2), NO_(2)` pulls the electrons of the O-H in `HNO_(3)` more strongly than NO pulls the electrons of the O-H bond in `HNO_(2)`. In other words, `HNO_(3)` releases a proton more readily than `HNO_(2)` and hence `HNO_(3) or N_(2)O_(5)` is a stronger acid than `HNO_(2) or N_(2)O_(3)`.