What mass of water is produced from the complete combustion of 2.40x10-3g of methane?

The balanced chemical reaction for the combustion of methane is given below.

                            CH4 + 2O2 --> CO2 + 2H2O
The masses of the substances involved in the reaction are calculated.

1 mol CH4
                          12 + 4(1) = 16
2 mols O2
                        2 (2)(16) = 64
1 mol CO2
                        12 + 2(16) = 44
2 mols H2O
                          2(2 x 1 + 16) = 36

By using the dimensional analysis, the calculated values and the given above,
             mass of water = (2.4 x 10^-3 g methane) x (36 g water / 16 g methane)
                mass of water = 5.4 x 10^-3 g of water

They are all decomposition reactions.