calculate th molarity of 10 volumes of H2O2

20 volume means 1dm3 of the H2O2 solution will produce 20dm3 of oxygen, where:

H2O2 -> O2 + H2:

i.e 1:1 stoichiometry. At 298K and 1atm pressure, 1 mole of gas occupies 24.45dm3. So 20/24.45 = 0.818 mol. O2 produced, which means there were 0.818mol H2O2 in 1dm3. Therefore molarity is 0.818M at 298K