Question

Given the incomplete equation for the combustion of ethane, `2C_2H_6 + 7O_2 -> 4CO_2 + 6__`, what is the formula of the missing product?

Answer 1

`H_2O`, aka water.

Explanation:

In general any organic compound, when burned will generate an "oxide" of its atoms.

Carbon goes to `CO` or `CO_2` (the latter, if complete)
Hydrogen goes to `H_2O`
Nitrogen and Sulfur depend on the amount of `O_2` available. If the amount is big enough they'll go for higher oxidation states such as `NO_2`, `SO_3` but with less oxygen they'll settle for lower states like `NO` and `SO_2`

Another way to solve this is to count the atoms that aren't in the right side. If you do the math you'll see we're missing 12 Hydrogen atoms and 6 Oxygen atoms, dividing by the stoichiometric coefficient that was given, we have 2 Hydrogen atoms and 1 Oxygen atom together to make a compound, which is obviously water.