Question

a 500.0 g sample of h2o2 conatins 31.50% h2o2 by mass find the mass of water in the solution

Answer 1

31.5 g H2O2/(31.5 g H2O2 +68.5 H2O) = 31.5% solution by mass. That is the ratio for 100 g. Just multiply water by 5 to fine in 500 g.