How many grams of hydrogen peroxide are needed to produce 90 g of water?
H2O2 = H2O + 1/2O2
Calculate how many moles of water you have in 90 g of water
From the balanced equation, this is the same as the moles of H2O2 you need. To get the mass of H2O2, multiply moles of H2O2 by its molar mass.
This would be a grams - grams stoichiometry problem!