posted by Tired .
Diarsenic trioxide is the number one source of arsenic; determine the mass of a sample of this extremely poisonous material containing 4.03x10^18 molecules.
I assume you mean 4.03E18 molecules OF As2O3.
mols in 4.03E18 molecules = 4.03E18/6.02E23 = ?
g As2O3 = mols As2O3 x molar mass As2O3.