Calculate the amount of H2O2: Using the balanced equation, calculate the number of moles of H2O2 present in the initial solution. Calculate the molar mass of H2O2 and determine the grams of H2O2 present in the initial solution. 5. Calculate the mass percent of hydrogen peroxide: Using the mass of H2O2 calculated above and the initial mass of the H2O2 solution, calculate the mass percent H2O2 in the initial solution. 6. Calculate the percent error: The accepted value for the concentration of hydrogen peroxide that is readily available at grocery stores and pharmacies is 3% by weight.