Fundamental (lattice, rotational, and intermolecular) vibrations of the H2AsO−4 anion in (C6H9N2)H2AsO4 crystal are calculated using the correlation theorem based on the group theory. The correlation between anionic site of symmetry Cs and the factor group D2h of the crystal yields 12 modes for both lattice and rotational vibrations. The infrared and Raman spectra of these modes do not coincide. Addition of two hydrogen atoms to AsO−4 ion yields two As—OH bonds in the H2AsO−4 anion. As a result, the molecular symmetry is reduced from Td to C2v. The free H2AsO−4 anion having C2v symmetry gives in total 15 fundamental normal vibrations. Under the crystal field splitting effects, the number of intermolecular vibrations for the anion in infrared and Raman spectra is calculated to be 56 active vibrations. The calculated fundamental vibrations manifest themselves as the main features in an experimental infrared spectrum.