Balanced chemical equations are balanced not only with respect to charge and number of each kind of atom but also with respect to weight, or more correctly mass. The periodic table lists these atomic weights :C=12 (exactly), H=1.01, O=16.00. So wwe can identify each atomic symbol with an appropriate mass:
CH4 + 2O2
12 + (4 x 1.01) + 2(16.00 + 16.00)
^
16.04 + 64.00
^
80.04
+
CO2 + 2H2O
12 +(2 x 16.00) + 2[(2 x 1.01) + 16.00]
^
44.00 + 36.04
^
80.04
Thus, 16.04 atomic mass units (amu) of CH4 react with 64.00 amu of O2 to produce 44.00 amu of CO2 plus 36.04 amu of H2O. Or 1 mole of methane reacts with 2 moles of oxygen to produce 1 mole of carbon dioxide plus 2 moles of water. The total mass on each side of the equation is conserved:
16.04 + 64.00 = 80.04 = 44.00 + 36.04
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