Question

A vessel containing 32 gram of CH4 gas was partially evacuated to remove 6•023×10^23 molecules of CH4 .How many grams of hydrogen and carbon are left in vessel?

Answer 1

the atomic weights of hydrogen and carbon are approximately 1 and 12, so the molecular weight of methane is 16. You have two moles worth of methane initially, and you have removed one avogadro's number of methane molecules, i.e., one mole of methane is gone. So one mole is left in the container, or 4 grams of hydrogen and 12 grams of carbon.

if you want to carry as many significant figures in the answer as you have shown in Avo's number, you'll have to use more exact atomic weights, 12.011 and 1.0079 for carbon and hydrogen, respectively.