Oxygen is generated by decomposition if hydrogen peroxide by the following reaction:
2 H2O2 -> 2 H2O + O2.
Thus, 2 moles of peroxide generates 1 mole of oxygen.
There is some confusion in the given question. I assume we are mixing 440 gm of peroxide in each liter and have 6 lt solution, that is, overall we have 6 x 440 gm = 2640 gm of peroxide.
At STP or standard temperature and pressure conditions, an ideal gas will occpy 22.4 liters of volume. Let's begin by converting the given quantity of peroxide into moles.
The molar mass of peroxide is 34 gm per mole. Thus, we have 2640/34 = 77.65 moles of peroxide.
Using stoichiometry, each mole peroxide will generate half mole oxygen. Thus, a total of 0.5 x 77.65 = 38.825 moles of oxygen will be generated. And this is equal to 38.825 x 22.4 lts = 869.68 liters of oxygen.
Thus, for 6 lts of solution containing 440 gm hydrogen peroxide per liter, we will obtain almost 870 lts of oxygen.
Hope this helps.