The molecular formula for mustard gas is C₄H₈SCl₂ the molecular mass of mustard gas - 159.09 g/mol atomic mass of H is 1 g/mol The mass of H in 1 mol of compound - 1 x 8 = 8 g this means that in 159.09 g of compound , 8 g of H present mass percentage of hydrogen = 8 g/ 159.09 g x 100% mass percentage of hydrogen = 5.03%