  We study the formation of water and methanol in the dense cloud conditions to
find the dependence of its production rate on the binding energies, reaction
mechanisms, temperatures, and grain site number. We wish to find the effective
grain surface area available for chemical reaction and the effective
recombination timescales as functions of grain and gas parameters. We used a
Monte Carlo simulation to follow the chemical processes occurring on the grain
surface. We find that the formation rate of various molecules is strongly
dependent on the binding energies. When the binding energies are high, it is
very difficult to produce significant amounts of the molecular species.
Instead, the grain is found to be full of atomic species. The production rates
are found to depend on the number density in the gas phase. We show that the
concept of the effective grain surface area, which we introduced in our earlier
work, plays a significant role in grain chemistry. We compute the abundance of
water and methanol and show that the results strongly depend on the density and
composition in the gas phase, as well as various grain parameters. In the rate
equation, it is generally assumed that the recombination efficiencies are
independent of the grain parameters, and the surface coverage. Presently, our
computed parameter $\alpha$ for each product is found to depend on the
accretion rate, the grain parameters and the surface coverage of the grain. We
compare our results obtained from the rate equation and the one from the
effective rate equation, which includes $\alpha$. At the end we compare our
results with the observed abundances.
