At stp conditions, 0.25 mole of co 2 (g), h 2 (g), nh 3 (g) will:

 1 mole of any substance contains 6.02 x 10^23 particles. 

Hence 0.25 mole of any has will contain 0.25 x 6.02 x 10^23 molecules of that gas

They cannot contain the same no. of atoms as 1 molecule of CO2 contains 3 atoms, 1 molecule H2 contains 2 molecules, and 1 molecule of NH3 contain 4 atoms

1 molecule of any gas will occupy 22.4 L at STP

hence 0.25 mol of all of these gases accupy the same volume

all of these molecules have different molar masses. thus their g.f.w cannot be same

Your answer is that they will occupy the same volume, and the same number of molecules

Ibelieve its because the stove is a cleaner burning fuel