The missing coefficient in this chemical equation will be the number that makes the number of Carbons and Oxygens equal on both sides. To start, we can tally what we have so far and then we can figure out what number we need.
2 C8H18 + 25 O2 --> ____ CO2 + 18 H2O
On the left, we have 16 Carbons, 36 Hydrogens, and 50 Oxygens.
On the right, we have 36 Hydrogens and 18 Oxygens (as of now).
This means we need 16 Carbons and 32 Oxygens on the right side of the equation to make the individual numbers equal; this can be accomplished by putting 16 as the CO2 coefficient (this adds 16 Carbons and 32 Oxygens). To check yourself, re-count the number of each atom on each side.
Left: 16 Carbons, 36 Hydrogens, 50 Oxygens
Right: 16 Carbons, 36 Hydrogens, 50 Oxygens
The numbers on both sides are equal, so 16 satisfies the CO2 coefficient and the chemical equation is now balanced. We're done!
Hope this helps!
