if your lab partner determined that there were 4.030 x 1020 atoms of H in an unknown sample of C2H4Cl2, how many milligrams would the unknown sample weigh?
4.030E20 atoms H/4 = # molecules C2H4Cl2.
mols C2H4Cl2 = #molecules C2H4Cl2/6.02E23 = ?
1 mol C2H4Cl2 has a mass of what? (Isn't that 2*12 + 4*1 + 2*35.5) so that x # mols = grams. Convert to mg.