$C{H}_4+C{l}_2\stackrel{hv}{\to }C{H}_{3}Cl+HCl$ , to obtain high yields of Methyl chloride  , the ratio of methane: Chlorine  must be

The amount of mono halogenated product obtained during the free radical substitution reaction. Should be carried out in the presence of excess of Alkane. Due to this the population of methane will be more and collision/reaction with methane is high, resulting in more percent of Methyl chloride as follows.