You are to determine the number of molecules of #NH_3# .
One molecule of #NH_3# contains 3 atoms of Hydrogen.So to produce one mole of #NH_3# we require #3/2molH_2# The following equation also supports that
#N_2+3H_2->2NH_3#
So 1 mole #H_2# produces #2/3mol NH_3#
Hydrogen taken #=7.98xx10^-4g#
#=1.64xx10^-4gxx1/(2.016g/"mol")#
where molar mass of Hydrogen #=2.016g/"mol"#
So no.of moles of #NH_3# produced
#=2/3xx1.64xx10^-4gxx1/(2.016g/"mol")#
So no.of molecules of #NH_3# produced
#=2/3xx1.64xx10^-4gxx1/(2.016g/"mol")xx6.023xx10^23"molecule"/"mol"#
#~~3.27xx10^19#molecules