Small quantites of hydrogen peroxide can be prepared by the action of an acid on an alkaline earth metal peroxide, such as barium peroxide.
BaO2 (s) + 2HCl (aq) -> H2O2 (aq) + BaCl2 (aq)
What mass of hydrogen peroxide should result when 15.0g of barium peroxide is treated with 25.0 mL of hydrochloric acid solution containing 0.0272g of HCl per mL? What mass of each reagent is left unreacted?
So you have 15.0g BaO2 and 0.0272 g/mL x 25.0 mL = grams HCl. This is a limiting reagent and you must first convert grams to mols. Here is a worked example of a limiting reagent problem that starts with grams.
http://www.jiskha.com/science/chemistry/limiting_reagent_problem.htmlposted by DrBob222