  We construct models of molecular clouds that are considered as ensembles of
transient cores. Each core is assumed to develop in the background gas of the
cloud, grow to high density and decay into the background. The chemistry in
each core responds to the dynamical state of the gas and to the gas-dust
interaction. Ices are deposited on the dust grains in the core's dense phase,
and this material is returned to the gas as the core expands to low density.
The cores of the ensemble number typically one thousand and are placed randomly
in position within the cloud, and are assigned a random evolutionary phase.
  The models are used to generate molecular line contour maps of a typical dark
cloud. These maps are found to represent extremely well the characteristic
features of observed maps of the dark cloud L673, which has been observed at
both low and high resolutions. The computed maps are found to exhibit the
general morphology of the observed maps, and to generate similar sizes of
emitting regions, molecular column densities, and the separations between peaks
of emissions of various molecular species. The models give insight into the
nature of molecular clouds and the dynamical processes occurring within them,
and significantly constrain dynamical and chemical processes in the
interstellar medium.
