The balanced chemical equation for the complete consumption of methane is as given below:
CH4+2O2→CO2+2H2O
3.01×1023 water molecules corresponds to 3.01×10236.02×1023=0.5 mol.
They will be produced from the combustion of 0.5×12=0.25 mol of methane.
This corresponds to 0.25×16=4 g of methane.