The information in the Inorganic Crystal Structure Database (ICSD) can be used to determine the structures of new compounds by analogy; this database is thus a useful tool for practical problem solving. A model for the structure of (NH4)Fe(CO3)(OH)2 [Cmcm, a = 6.6154 (6), b = 12.0639 (10), c = 6.0263 (5) Å, Z = 4] was found by searching the ICSD for compounds with C-centered orthorhombic cells and containing C, H, N and O. [Al(H2O)6]2[Al(H2O)5(SO4)]2-(H3O)2(SO4)5 [P\bar{1}, a = 12.5881 (5), b = 13.0726 (5), c = 7.3354 (2) Å, α = 108.162 (2), β = 70.536 (2), γ = 112.658 (2)°, Z = 1] is isostructural to a Cr compound of the same stoichiometry, which was located by searching on the formula type ANX = A4B7X52. Suitable models for MgCl2(H2O)4 [P21/c, a = 5.9001 (9), b = 7.2709 (7), c = 8.4199 (7) Å, β = 111.007 (15)°, Z = 2], MgCl2(H2O)2 [C2/m, a = 7.3886 (29), b = 8.5498 (24), c = 3.6488 (17) Å, β = 98.96 (3)°, Z = 2], and MgCl2(H2O) [Pnma, a = 8.899 (7), b = 3.6339 (20), c = 11.398 (8) Å, Z = 4] were located by combining searches on chemistry and formula type. Quantum-chemical calculations were performed on all five structures to locate the H-atom positions and to obtain quantitative understanding of the hydrogen bonding important in all the structures.