When ethane reacts with oxygen to form carbon dioxide and water. When 10 molecules of ethane react with 20 molecules of oxygen, what is the composition of the FINAL mixture?

Answer 1

I would balance the equation:

C2H4+3O2 >> 2CO2 + 2H2O

So it looks to me as if you did not have enough oxygen, which means, only 2/3 of the ethane was consumed.

So you have an issue, as molecules only occur in whole number.

I see
4 molecules C2H4 ; six consumed
2 molecules O2 left; 18 consumed
CO2; six made
H2O; twelve made