# Why do we know that in the balanced chemical equation C + O2 ---> CO2, 1 g of C will not react exactly with 1 g of O2?

$C$ atoms weigh 12 atomic mass units. ${O}_{2}$ molecules weight 32 atomic mass units. So, in 1g of carbon atoms there will be lots more carbon atoms than there are molecules of oxygen in 1g of oxygen.