How many molecules are present in 1.0 kg of hydrogen gas?

##3.011 xx 10^(26)## molecules

To figure out the number of molecules of hydrogen in 1.0 kg of hydrogen gas we need first to find the number of mole (##n##), then:

##"no. of molecules" = "no. of moles" xx "Avogadro's number"##

the number of mole n = ##(m)/"MM"##, where MM is the molar mass which is equal to 2.0 g/mol. And m = 1 kg = 1000 g.

Therefore,

##n = (1000)/"2.0" = "500 mol" = 5.0 xx 10^2 "mol"##

Therefore,

##"no. of molecules" = 5.0 xx 10^2 cancel("mol") xx 6.022 xx 10^(23)"molecules"/cancel("mol")##

##"no. of molecules" = 3.011 xx 10^(26) "molecules"##