  It is important to understand the origin of molecular line intensities and
chemical composition in the molecular-cloud scale in the Galactic sources
because it serves as a benchmark to compare with the chemical compositions of
extragalactic sources. Recent observations of the 3-mm spectra averaged over
the 10-pc scale show similar spectral pattern among sources for molecular lines
HCN, HCO$^+$, CCH, HNC, HNCO, c-C$_3$H$_2$, CS, SO, N$_2$H$^+$, and CN. To
constrain the average physical property emitting such spectral pattern, we
model molecular spectra using a time-dependent gas-grain chemical model
followed by a radiative transfer calculation. We use a grid of physical
parameters such as the density $n=3 \times 10^2 - 3\times 10^4$ cm$^{-3}$, the
temperature, $T=10-30$ K, the visual extinction $A_{\rm V} = 2,4,10$ mag, the
cosmic-ray ionization rate $\zeta = 10^{-17} - 10^{-16}$ s$^{-1}$, and the
sulfur elemental abundance $S/H = 8\times 10^{-8} - 8\times 10^{-7}$.
Comparison with the observed spectra indicates that spectra are well reproduced
with the relatively low density of $n=(1-3) \times 10^3\,$cm$^{-3}$, $T=10\,$K,
$\zeta = 10^{-17}$ s$^{-1}$, and the short chemistry timescale of $10^5$ yrs.
This short chemistry timescale may indicate that molecular clouds are
constantly affected by the turbulence, and exposed to low-density, low $A_{\rm
V}$ regions that "refreshes" the chemical clock by UV radiation. The relatively
low density obtained is orders of magnitude lower than the commonly-quoted
critical density in the optically thin case. Meanwhile, this range of density
is consistent with results from recent observational analysis of
molecular-cloud-scale mapping.
