Question

What number should be substituted for the question mark to balance the equation and why is that the correct number?

Answer 1

The combustion of ethane results in the balanced equation 2C2H6 + 7O2 -> 4CO2 + H2O + energy because we see that after the combustion, the reactants have 8 + 6 = 14 Oxygen atoms in total (8 from 4CO2, 6 from 6H2O). Therefore, since there are 2 atoms of Oxygen in oxygen gas, we have 7O2 molecules in the beginning to hold the ratio true.

Answer 2

To balance an equation, we need to make sure the amount of atoms on the reactants and the products. 

How much oxygen's are on the reactants side? 

Lets count, 4*2 = 8  and 6, so 8+6 = 14

Now, start to predict the number. 

14/2 = 7

Answer: 7 is the correct answer