We present an unbiased spectral survey of one of the most chemically rich hot molecular cores located outside the Galactic Center, in the high-mass star-forming region G31.41+0.31. In this first paper, we discuss the detection and relative abundances of the three isomers of C_2_H_4_O_2_: methyl formate, glycolaldehyde, and acetic acid. Observations were carried out with the ALMA interferometer, covering all of band 3 (~32GHz bandwidth) with an angular resolution of 1.2"x1.2" (~4400aux4400au) and a spectral resolution of ~0.488MHz. The transitions of the three molecules have been analyzed with the software XCLASS to determine the physical parameters of the emitted gas. The comparison with chemical models in literature suggests the necessity of grain-surface routes for the formation of methyl formate in G31, while for glycolaldehyde both grain-surface reactions and gas-phase chemistry could be able to explain the observations.