The asymmetric unit of the title salt, C(7)H(9)N(2)O(2) (+)·Cl(-), contains two independent cations and anions. In the crystal, each N-methyl-4-nitro-anilinium cation is linked to two Cl(-) anions via N-H⋯Cl hydrogen bonds. π-π stacking is observed between the benzene rings of adjacent cations [centroid-to-centroid distances = 3.7684 (14) and 3.7917 (7) Å].

In the title hydrated salt, C(7)H(9)ClN(+)·Cl(-)·0.5H(2)O, the water O atom lies on a crystallographic twofold axis. In the crystal, the monoprotonated 4-chloro-benzyl-ammonium cation forms N-H⋯Cl and N-H⋯O hydrogen bonds and the water mol-ecule forms O-H⋯Cl hydrogen bonds, generating layers lying parallel to the bc plane.

In the crystal structure of the title salt, C(10)H(14)Br(2)N(+)·Cl(-), the organic cations and chloride anions are linked into one-dimensional chains parallel to the a axis by N-H⋯Cl and N-H⋯Br hydrogen bonds.

The structure of the title compound, C(5)H(5)N(2)O(2) (+)·Cl(-), is composed of chloride anions and 4-carboxy-pyridazin-1-ium cations. Chloride anions bridge the cations via O-H⋯Cl and N-H⋯Cl hydrogen bonds to form ribbons. The latter, linked by van der Waals forces with lengths in the range 3.254 (2)-3.497 (2) Å, form coplanar layers. Very weak inter-actions operate also between adjacent layer...