  We introduce the concept of a class of equivalence of molecular clouds
represented by an abstract spherically symmetric, isotropic object. This object
is described by use of abstract scales in respect to a given mass density
distribution. Mass and average density are ascribed to each scale and thus are
linked to the density distribution: a power-law type and an arbitrary
continuous one. In the latter case, we derive a differential relationship
between the mean density at a given scale and the structure parameter which
defines the mass-density relationship. The two-dimensional (2D) projection of
the cloud along the line of sight is also investigated. Scaling relations of
mass and mean density are derived in the considered cases of power-law and
arbitrary continuous distributions. We obtain relations between scaling
exponents in the 2D and 3D cases. The proposed classes of equivalence are
representative for the general structure of real clouds with various types of
column-density distributions: power law, lognormal or combination of both.
