Question

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?

Answer 1

Because carbon atoms are not the same mass as oxygen molecules.

`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.

The balanced equation tells us that 1 atom of carbon reacts with exactly one molecule of oxygen, so we can scale that up and say that 12g of carbon will react with 32g of oxygen. That is clearly not the same ratio as 1g reacting with 1g.