Two molecules with the same structural formula must have the same types and numbers of atoms of each element. In other words, they have the same chemical composition. However, it's important to note that even if two molecules have the same structural formula, they can have different spatial arrangements or arrangements of atoms in space, which is referred to as stereoisomerism. Therefore, while the structural formula provides information about the connectivity of atoms in a molecule, additional information may be required to fully describe their properties, such as the arrangement of atoms in three-dimensional space.