Reaction
CH₄ (g) + 2O₂ (g) <=> CO₂ (g) + 2H₂O(g)
assume at STP ( 1 mole gas = 22.4 L)
for 20 L, moles of methane:
= 20 : 22.4 = 0.893
moles of water = 2/1 x mol methane = 2/1 x 0.893 = 1.786
mass of water = mol x molar mass = 1.786 x 18 g/mol = 32.148 g