a 251.7-mg sample that contains a mixture of na2so4 and mgcl2 is dissolved in water and treated with bacl2, precipitating the so42- as baso4. the resulting precipitate is isolated by filtration, rinsed free of impurities, and dried to a constant weight, yielding 333.5 mg of baso4. what is the %w/w na2so4 in the sample?